The requirements vary from state to state, but in general they follow a common path.  The process is not short, so be prepared to make an investment in your time before even having a chance for a placement in your home.  Also, be aware that a large part of the introduction and initiation is designed to scare you.  Social services really want to make sure that those they train are in for the long haul.  Also, it is really important to understand that just because you start looking at fost/adoption as a way to build your family does not mean you are committed to that path.