The bit is your communication link to your horse, along with the reins, pressure from your legs and body position. Depending on your style of riding and your experience level as well as the condition of your horse's mouth, different bits will be more appropriate than others. Generally most horses that have not been mistreated will have what is called a "soft mouth" which means that their mouth has not be injured by misuse of a bit or cruel treatment by an uncaring rider. Horses that have been misused by cruel and excessive use of a bit are known as "hard mouthed" and require specialized combinations of bits to control the horse.
Thankfully there are very few horses that have been mistreated and most horses have a soft mouth. The most commonly recommended type of bit will both ensure that the rider has control as well as provide protection to the horse's mouth. Getting the right sized bit is essential in making the bit comfortable for the horse and prevent pinching the sensitive skin at the side of the mouth. In general most light horse breeds will use a five inch bit, which is the measurement across the bit. Some horses will have a wider or narrower mouth, so a good rider will always measure across the horse's mouth to the outside of the lips, then add an additional ½ inch. If you start with a five inch bit you can then see if you need to go wider or narrower, or you can try a variety of bits and measure which one is the best fit. Keep in mind that the bit should have ¼ of an inch on either side of the outside of the horse's mouth to fit properly. A string can be used and measured if you don't want to have to try an assortment of bit sizes. Just side the string across the horse's tongue in the place the bit would normally rest and measure out ¼ inch on either side to get the final bit size.
Bits Tools
English and western bits are different, as are bits for racing, dressage and other types of specialty events. The most common bit in both English and western styles of riding is the snaffle bit, which is also the most gentle on the horse's mouth. A snaffle bit is two bars that connect in the middle, with the western snaffle also known as the snaffle curb since it will have two additional "curb" pieces on either side. Snaffle bits are great for novice riders as even if the rider pulls harder than necessary on the bit the horse's mouth will not be injured. Horses that have hard mouths may need additional types of bits such as full curbs, rollers, Kimberwick and Pelham bits, depending on the style of riding. Since these bits can and will cause damage to the horse if used incorrectly, they are not recommended for riders without significant experience.
Always clean your bit after each ride to prevent the build up of debris, goop and slobber on the bit. It is a good idea to have a spare bit around that can be buckled into the headstall of the bridle and used while the other bit is being thoroughly cleaned. Soaking the bit in clear, warm water for a hour will typically soften any dried on material which can then be removed with a stiff bristle brush and a good scrubbing.