The requirements are different for different states. We might be able to help better if you let us know in which state you'd like to practice. Here's a list of all the Licensing Boards for the US:
http://www.asppb.org/about/boardContactStatic.aspx
Each website should have the requirements. It may depend on where you got your degree in the first place, as well. You may need to write the EPPP (Examination for Professional Practice in Psychology).