If you want your horse to go where you want to go when you are riding him, then you will need to use one essential piece of equipment, the bit. The bit is basically a metal bar that sits in the horse's mouth and hooks to the two sidepieces of a bridle. When you tug gently on the reins, the bit applies pressure to the horse's mouth, so he knows that you want to stop or turn. Of course, there are two distinct bit styles, one for Western riders and one for English riders. Within these styles, there are also different bits, since sometimes a certain bit does not work as well as another one.
No matter which type of bit you decide to buy, you will need to be sure that it fits your horse's mouth properly. You will need to measure the width of your horse's mouth at the spot where the bit rests and add a half inch to this measurement to get the proper fit.
Although you could try to stick a measuring tape in his mouth to measure the width, it is easier to put a few different bridles on him and see if any of the bits fit so that there is about a quarter inch of the bit is showing on either side. Once you find a bit that fits properly, you can remove the bridle, measure the bit, and ask your tack shop for a bit with that measurement. Of course, if this all sounds way too complicated, you can simply ask for a five inch bit and there is a pretty good chance it will fit your horse, unless you have a breed like the Percheron or a Miniature.
For most people, whether they prefer English or Western riding, the snaffle bit is the only style that is necessary. The snaffle is the gentlest bit, so is the ideal choice for any horse that does not have a mouth that has been hardened by bad riders. Of course, this also makes the snaffle bit ideal for beginning riders, since they have a tendency to accidentally tug too hard on the reins or pull on them for balance. The standard English snaffle bit is simply two bars of metal linked together to form a bit long enough to fit comfortably in the mouth of your horse, with about a quarter of an inch extending from each side. The Western snaffle bit is actually called a snaffle curb, because another piece of metal extends vertically from the sides of the snaffle on either side.
For horses with bad habits or mouths that have been hardened by heavy handed riders, there are several other options. However, do not purchase one of these bits unless a trainer recommends one for your horse, or you may ruin his mouth. These bits are the standard curb and curb with roller bits for Western riders and the Pelham and Kimberwick bits for English riders.
Of course, since a bit can get pretty messy from a combination of horse saliva and bits of grass, you may also want to buy a spare bit. Although tack should always be cleaned after you care for your horse and return him to his stall, sometimes it gets neglected. A spare bit can sometimes mean the difference between being able to exercise your horse and having to spend the day soaking gunk off of your only bit.