Answer:
Step One: Find Out Whether You Are Eligible for U.S. Citizenship
The first question is whether you have a U.S. green card (lawful permanent residence). With few exceptions, you must obtain a green card before you become eligible to apply for citizenship. So if you haven't yet reached this point, learn about your eligibility by reading Can You Get a U.S. Green Card?.
As a lawful permanent resident, you must meet additional requirements in order to be eligible for U.S. citizenship. These concern the length of time you've spent in the U.S. as a green card holder, your good moral character, your ability to pass a test in English, and on U.S. history and government, and more.
To check on whether you are eligible, see Who Is Eligible to Become a Naturalized U.S. Citizen?
Now would also be a good time to begin improving your English, if need be, and studying the possible questions that appear on the exam. There are 100 total questions the USCIS examiner can draw from, and you'll need to be able to answer six out of 12 correctly in order to pass.