The path to acquire American citizenship is through naturalization. It is a complex process and if you went through a straightforward green card process and have been a law-abiding resident in the meantime with nothing out of the ordinary, there is no need for an attorney. You can file your application yourself. If you like the do-it-yourself (DIY) approach, I have plenty of help for you. You can download this Kindle ebook on Amazon -- DIY Guide to Naturalization-- (don't worry if you do not have a Kindle reader because Amazon has a free software to read ebooks on a computer or iPad).
Here are some useful articles on naturalization:
Completed sample of USCIS Form N-400
Documents needed for naturalization application
What happens during the biometric appointment
Experience of Naturalization test and interview
How to prepare for the citizenship oath ceremony