That's a lot of questions lol. I'll start by saying take your time researching anything you want to do. Just go one modification want at a time and learn about it. I'm still learning things myself and that's kinda the way I've been doing it. Why search ebay for questions . Search here or just good old google, which a lot of times takes you here lol.
Cold air intake , exhaust modifications and a tune are the first three mods generally done when your starting out.
Headers are a part of the exhaust system and a cam is a part of the motor.
There's going to be different opinions on what modification one person likes compared to another , there will be different reasons for that. Financial situation, the sound they are looking for, hp gains, handling on the road or track and so on. Read , read , read lol.
If you can find or someone can post it for you. Find a break down diagram of a exhaust system. I have seen it on here before but cant recall which thread. That will help .
PBM GT W/SPORT PACK
Pat G tune
EXHAUST: BADLANDZ CUT OUTS W/ X PIPE AND RESONATOR DELETE
KOOKS 1 7/8 mids and HFC's (coated)
PONTIAC EMBLEMS REMOVED. HOLDEN TRUNK LOGO ADDED.