First of all, it almost always takes a Green Card.
In most cases, you must have resided in the USA for five years. During that time, you must not have made trips of more than six months to one year out of the USA (six months can be harmful, one year almost always will disrupt your residency).
You must also be of good moral character and not have any major criminal record - beware, even a DUI conviction can damage your chances at citizenship.
Finally, you must be able to read, write and speak English, pass a civics test and an interview, and take an oath.
This is a very short version; there are quite a few exceptions.
