What I'd do is read through them all and go from there. STeve:'s points are right on. One point I'd like to add is are you adding this door in a load-bearing wall??! In short, this project could be over your head. BUT, don't let it discourage you. You can have this job bid where you rip out the drywall (per the contractor's instructions), let him do the install, and you do the finish work (re-hang drywall, spackle, prime+paint.) You could certainly save some $$$ doing it this way.
As already stated, if you have more ?'s, post up! My best to ya and hope this helps.