CASPA is the PA school equivalent of AMCAS. It is an online centralized service that allows you to apply to as many schools as you want simultaneously. However, not all programs are CASPA participants.
The application cycle starts at the end of April, and continues basically until the following April, but most schools' deadlines are sometime in the fall. When you select the programs to which you would like to apply, CASPA will display each program's deadline next to its name, and it will also send you emails as program deadlines approach.
Be sure to allow yourself plenty of time to complete the CASPA application, as it can take many hours to get all of your information in there. It can also take a few weeks for your transcripts to arrive, and your application will not be sent to the schools until all materials have been received at CASPA.
CASPA does not accept GRE scores; those must be sent directly to the schools where you are applying. Institution codes for each program can generally be found on that program's website.
Because it is free to open an account, it might be a good idea to make yourself a login just to have a look around and familiarize yourself with the site.
Most of the sections are quite self-explanatory, and I strongly encourage you to read through the instructions for each section if you have any doubts about how to fill it out.
Coursework
Perhaps the most irksome thing about CASPA is the fact that you must input every college course that you have ever taken, whether or not you completed or retook it. Luckily enough, BYU uses the same credit hour and grading system as CASPA, so you don't have to worry about converting credit hours and grades unless you have transfer credits from another university.
There is no place to input AP credits, so double-check with the programs (not CASPA) that you have still met their admissions requirements with the AP credits.
To complete your application, you are required to get official transcripts from each college attended sent directly to CASPA. For BYU transcripts, just go to the records office in the ASB to request the transcript to be sent directly to CASPA. For other institutions, please contact their records offices directly to see what their policies are.