If you are getting ready to start PPL it would be the Private written which you study the textbook and PP Test Guide and watch the chapters on DVD that they sent you and you can take practice tests at a site like exams4pilots.org.
I was told I could schedule mine at ATP before I start my official class date or just come in and take it that Monday. I think someone also mentioned you could take it with a local FBO and get it signed or something dont remember.
It depends what program you are starting. If you are starting the private course, then just take the private exam. If you want to take it before you go, go to a FBO with a couple practice tests in hand with scores above 85 or 90 and try and talk an instructor into signing you off to take the test. ATP will reimburse you for up to $90 for the test.
If you are going to the career program, then take the IRA and FII atleast.