Obviously, it is much easier if you start with boots that fit as well as possible. We have just covered going to the right people.
Athletics training books advise that you never wear used footwear, because moisture makes them good ways of carrying infection. But at the beginning stages of learning to skate, you may not yet what type of skating you will do, and therefore don't know yet what boot you need. It is also possible you will decide that you do not like skating, or that your body is extremely unsuitable to it. If you buy used boots, be sure to spray in a disinfectant (e.g., Lysol), and an anti-fungal spray designed to disinfect Athletes' Foot. That won't cover everything, but is a good start.
Making boots is not an exact science. No matter how carefully you are fit or the boots are made, even custom boots often fit poorly. So the best bet is to go to a store with a large selection, and try pairs to find one that fits well. In fact two boot pairs of the same model and size will not fit the same.
If nothing comes close should you consider custom boots.
Rental boots are used boots. Obviously the rinks that rent boots to you would not want you to do the modifications discussed in this document. But you can certainly use socks that are higher than the boot - a good idea anyway, but it helps prevent getting foot diseases from a used boot. And you can still create your own insole. Don't damage a rental boot by removing an old one if is glued in, though you can put your new one on top. Be sure to restore the boot to original condition before returning it. Or use the paper towel stuffed sock temporary modification method. Personally, I would also want spray in disinfectant and anti-fungal spray. It is often possible, if you get to the rink early, to request a specific numbered pair, so you can consistently get the same fit.
Rental boots are usually ordered quite wide, which means they fit most people poorly.
Rental boots see heavy use, and change in shape somewhat by being used by prior customers who they fit too loosely, or who don't lace tightly, so they differ from pair to pair even more than new boots. Blades are often mounted and sharpened too quickly, so they differ from pair to pair a lot too. It is quite typical that you will find it impossible to balance on one rental pair, but can balance fine on another. That emphasizes the importance of getting a single consistent pair.
In the end, if you skate enough, you will probably decide it is worth getting your own pair, to get consistent fit, balance and edges, and so you can make your own modifications. If you buy too cheap, and get boots so soft they don't support your feet, they will be unsafe, and will quite likely lead to injury. As of the year 2010, a good pair of rental figure skating boots, with blades, cost about $200 (U.S. Dollars), new. You should buy something of similar quality. Perhaps you can go as low as $100, though that is pushing it, if you won't be too aggressive. Of course, used boots are usually somewhat cheaper, but should be in the same quality range, and shouldn't be broken down (e.g., there should be no strong creases in the leather). If you aren't willing to spend that, you may be better off staying with rentals.
Everything I say in the rest of this section applies to both feet. Be sure to test both boots. Both your feet and any given pair of boots differ somewhat from one foot to the other. Be sure to try your boots in very thin socks, like most people skate in.
Make sure all fits are down with your heel fully jammed into the heel of the boot. After you put the boot on, it helps to use your foot weight to pound the back of the boot blade onto the floor. Some foot shapes may not otherwise move all the way back.
In the best of all possible worlds, the boot will be snugly comfortable and touching with approximately equal pressure all over your feet, when the lace is tied tight enough, which when the lace is tied tight enough, and in front of and behind the leg above the ankle, where many people, especially freestyle skaters, want extra space. A boot that already happens to fit your foot doesn't need to be laced as tight as one that needs to be bent to conform to your foot, but few people will find such a perfect fit in stock boots. Some boots come with laces that are hard to grab tightly. Depending on the lace hole size, you may be able to get a wider lace that is easier to grab. Some brands and models of beginner boot use buckles, like ski boots do, that are easier for some people to make tight than laces. Your foot should not be able to slip anywhere within the boot, even if you push the boot strongly forward and back against the floor (or ice), or push your toes or heel side to side. You should definitely not be able to bend or rock the top of your foot so it stops being snug against one or both sides. (I used to claim you shouldn't have much pressure in front of or to the sides of your toes, to avoid hammertoes and bunions - but a podiatrist corrected me - a normal amount of pressure there is OK for most people.) All such motions lead to a lack of control, as well as possible injury. However, the chance that a stock boot will perfectly match your exact foot shape everywhere is quite low. As shall be discussed, it is quite possible to make some extra space by removing or replacing the insole, or to fill empty spaces within the boot, and you are better off with a boot that starts out a little loose, if you are willing to make those modifications, than with one that starts out tight enough (with the insole removed) to hurt anywhere. As will be discussed, some people prefer to have a little open space, or lessened pressure in front of and behind the foot, but just at the very top of the boot, so they can more easily flex and point their ankle. As will be discussed, they will probably require extra padding around the top of their feet to protect their skin from blisters. The lessened forward and back pressure is important for high level freestyle and pairs skaters who jump, and may not be needed for beginners who don't.
A nice soft plush lining is a luxury many people love. It can compensate for a slight misfit and helps the boots "breath", to get rid of the sweat you will generate as you skate. Unlike thick socks, which will slip against your feet and likely create blisters as well as loss of control, the lining is locked in place and won't slip. Wearing two pairs of socks to prevent blisters is good for backpacking boots, but can lead to a loss of control and possibly injury in skates.
Also in the best of all possible worlds, your boot must both allow your complete foot (especially ankle) safe range of motion, without pain (e.g., without the top of the boot digging painfully into your feet) and to limit the range of motion to be within that range. You should be able to flex your ankle back, so your toe points towards your leg as much as you can, or so it points in the opposite direction towards the floor as much as you can. You should be able to bend your ankle sideways inwards and outwards (a doctor would say into pronation and supination) as much as you can. In fact, move your ankle into foot circles, so the toe moves in as large a circle as it can. The boot should not hurt or dig into your foot at any point within that range of motion, but as you approach the limits of safe motion, the boot should provide sufficient resistance to bending that your feet won't be forced into an unsafe position. Balancing on blades is more difficult than balancing in a normal shoe, so it is very possible to be accidentally pushed into an unsafe position, so this is very important. It isn't likely you will find a boot that perfectly matches this requirement; it is especially important that the boot provide enough resistance to prevent you from bending your angle sideways too far to be safe, inside or out (a doctor would say into excess pronation or supination). To some extant, a boot that digs in to your foot at its top because it is too rigid can be fixed by using various types of padding, if there is sufficient space there inside the boot, but I would view that as a very bad sign, as it will bruise. Such excess pressure or sheer (tendency to cut) can even burst your Achilles tendon, or bruise bone.
The range of motion just discussed, especially in the sideways direction, must be lessened when doing jumps, because the take off and landing provide so much extra pressure to sprain or strain your ankle that you need an extra margin of safety. Thus the prior paragraph applies better to beginners, and to ice dancers, than to higher level freestyle and pairs skaters. But we are discussing your first boots, in which you probably won't jump much. One of the most common mistakes beginners make is to buy boots that are too stiff, because they think that more expensive boots, designed for high level freestyle skaters, must be better. They never break in, are heavy, prevent their less developed muscles from moving enough, and cause pain in the long run. Don't make that mistake! However, I personally believe that if there is enough space in front of the boot above the ankle to fully point and flex your feet, you can use stiff boots - but be sure the sides are snug.
Some people with certain medical conditions, such as a tendency towards excess pronation or supination, may have to limit their motion in ways and to an extant than only a medically trained individual familiar with their case could advise.
As mentioned, each boot should not make you hurt at all, anywhere, anywhere within its safe range of motion. A boot might hurt because it is too tight - but, as discussed in succeeding pages, you can make a little extra space by removing or replacing the insoles. It might hurt because it is too tight at specific places on your foot. As we shall discuss, some amount of stretching and reshaping is possible, but if you need more than (very) approximately 1/10 inch (1/4 mm) of stretch, anywhere, the stretch probably won't be completely permanent, and will keep needing to be redone - expensive if you don't do it yourself, a nuisance if you do. It is particularly hard to stretch the toe area. I think it is easier and more permanent to fill extra space, using methods to be discussed here, than to stretch the boot, so I would prefer to start with a slightly loose fit, if I can't find a boot that is perfect. That statement is definitely heresy within the skating community, but that is because most skaters don't realize space can easily be filled with just a little time and effort.
If your feet are substantially different sizes or shapes, you may need to special order a "split size" pair. Most people can instead manage to fill some extra space inside one or both boots by adding tape in the right places.
Advanced figure skaters need a toe pick that is easier to reach by pointing the toe than is ideal for a beginner, who usually has trouble avoiding tripping over the toe pick. So the inexpensive beginner blades that usually come with new boots are better for beginners than the blades more advanced skaters use.