How can I get my tax back ? - international student?

Hey! I've participated in Work and Travel program in last two years. I did paid federal tax, but exactly I did not have to. ( as a international student I'm not oligated to do it) Now I'd like to get back what I paid.The problem is that I don't know how to do it. My knowledge about US taxation process is not impressive.I stilll have my W2 forms from my employer.Please help me!!!