A well fitting bit is integral to both a pony's comfort and the effectiveness of the trainer's rein aids regardless of the type or style of bit your pony wears. If a bit is not secured well, it can be both uncomfortable and confusing (since it may interfere with proper communication of rein signals) for the pony.
When fitting a bit, there are three main considerations: how wide is your pony's mouth (i.e. what is the opimal bit size, and how small a bit can he accomodate), how to attach it, and how tight to attach it.
The first step is to measure your pony's mouth. This will give you a starting point for what approximate size mouthpiece you should use on your pony. Of course, the style of the bit will also have a bearing on whether you should go up one or two sizes. For example, some bits can pinch the sides of your pony's mouth, and thus you should buy a bit 1/4" - 1/2 wider than the width you measure.
Fortunately, it's quite easy to measure your pony's mouth. There's a convenient tool that let's you size your pony for a bit. Here it is being used to size my mouth (which turns out to be 3 1/2" (as you can see in the photo below) for a non-pinching snaffle, or up to 4" for bits that can pinch):
The type of bit a pony wears will dictate how it is attached to his bridle and where the reins attach:
If you aren't precisely sure what bit you have, a snaffle is usually a good bet. Most ponies (and many bio-horses) wear some kind of snaffle. Gag bits (distinct from a "bit gag", which is a sort of generic term for any horse bit converted for use in BDSM) are probably the least common on human ponies since they require a unique bridle and rein setup.
The second consideration is how tightly to attach it. A properly fastened bit should sit behind the front teeth. In fact, it should be pulled back into the mouth far enough and snug enough to create two wrinkles in the skin at the corners of the mouth (as a general rule, but your pony's comfort should come first - there's no wrong way).
When choosing a bit keep in mind that whatever bit you choose should be comfortable for the pony to wear at that degree of snugness. This is generally only a problem for bits with more complicated mouthpieces (e.g. ports, thin wires, spiked, etc.), but it is something to keep in mind if you do decide on something other than a simple straight or broken mouthpiece.
Generally a little too tight is better than a little too loose (not a hard and fast rule by any means, but subtle reins signals can be lost if a bit is too loose), assuming your pony is okay with that.