The governing body of the PSAT, the SAT, and the AP exams is The College Board. Complete details of each exam, along with specific test dates and instructions for registration, can be found at The College Board's official website. I have listed the link on our Recommended Sites page. There is truly a wealth of information on this site for both students and parents, so plan to spend some time looking around while you're there.
Some recommended resources:
Barron's 2007-2008 ACT Test Prep
Cracking the PSAT
SAT Test Preparation