Every year, many people obtain their citizenship of United States through the naturalization application process. If you want to go through the process of Naturalization for US citizen, a good place to start naturalization is the US Citizenship Application web site.
Before you get approved for the US citizenship, you need to have an interview with the citizenship officer. To prepare for the USA citizenship test or interview is not a simple task, you need to get familiar with American Citizenship Test questions, mostly about US government, history, geography and culture. I recommend you use some free citizenship practice test software.