Bits work with pressure being placed on parts of the horse's mouth. The style of the bit determines where the pressure is placed. Pressure can be brought to bear on the lips, chin groove, tongue, bars and the roof of the mouth.
Snaffle bits are generally considered the mildest bit; every horse ought to be started in a snaffle bit. Curb and gag bits are the harshest bits and can be severe. It is important to find a bit that is appropriate to your horse's needs and one that fits.
A bit that does not fit your horse is ineffective and will give you more problems than what it is worth. If you do not know the size bit you need, ask horse owners to help you. Perhaps you could borrow a few different bits to try and fit. When you find one that does not hang loosely in your horse's mouth, or pinch up the corners of his mouth you have a fit.
You only need to see a quarter of an inch of bit outside of your horse's mouth. That is of the mouthpiece, the actual part that is placed in the mouth, not the part or parts that the reins attach to.
If you have a horse that is experienced in being ridden, you may need to purchase a bit that is not as gentle as a snaffle. Some riders have hard hands and with constant yanking and pulling on the reins, they "harden" a horse's mouth. A horse trainer will be able to tell you if your horse needs a stronger bit than a snaffle.
Do not buy a harsher bit just because you are having difficulty controlling your horse. There are several factors that could be attributing to the lack of control such as his diet and your inexperience.
A horse's mouth is going to harden over the years of being ridden. If you immediately change to a harsher bit without looking at other causes of lack of control, you are speeding up the process and could well be ruining your horse's mouth. Besides using your bit to direct your horse, you have to learn the correct seat and leg aids to communicate and control your horse.
When you find the correct bit for your horse, the true test will come when you ride him. Your horse should be comfortable, if he shows signs of resistance (acting different than usual and not in a good way) then you need to look further and find something to suit him better.
