You have to have resided in the United States as a permanent resident continuously for five years, you should have been physically present in the United States for half of the five years and you should be a person of good moral character. Also, you should at least be 18 years of age and legally competent to take an oath of allegiance to the United States.
the best step you can do is to go on line and search with the INS. and download information, forms, a list of fees and free dvd to learn all the requirements to become one. You can file it yourself just you will do a lot of reading.