chang_lee, you ask good questions, but there's one other key factor to consider: money. If you don't know anything about car audio equipment, then I'm also guessing you don't have a lot of experience in installing it. If that's the case, it's gonna be tough for you to do an upgrade without spending a decent amount of money (e.g., at least $1,000-1,500 and probably more), because even if you economize on equipment (which you wouldn't want to do if the goal is to upgrade!), you're going to spend alot on having a shop install it.
Also, search the ICE board and you'll see that many before you have asked this question, and there's lots of good info out there.
In a nutshell, to do a decent upgrade, you're going to need:
1) new front speakers
2) a subwoofer box or JL Audio Stealthboxes (subwoofers in custom enclosures to mount in the factory rear deck locations)
3) one large amp or two smaller amps to drive 1&2
4) the wire and hardware to install 1-3
Better picture coming!