there are several factors to consider.

One obviously enough for most is look, what do you want it too look like, you want to find something that you like, not your friends, though if you come to agreeance on some pieces its always a plus, especially when you smoke with them a lot.

2 is price, find your price range, there is no need to find a piece if you cannot afford it.

third-type, there are several types of bowls, think about what you want, carb/no carb, deep bowl/shallow bowl, long/short, and size of your holes at the mouth and the carb

four, material, glass? metal? acrylic? combinations? its up to you to decide what type of materials will fit your needs. Glass looks prettiest, but is fragile. Metal is sturdy, but heavy, and many people find that they have a "taste" one which i personally enjoy, plastic, and stone is heavy and not too fragile, though they can chip, and plastic is plasticy. Pick one or several.

tip-if you have hair that is in your face/on your face, make sure you get a piece that is long enough that you wont burn your hair when you smoke out of it, ive lost a few moustache hairs too a bic once or twice.