Well, in that case you have stock everything. I'll go over some basic lingo.
Your car is all stock. Radio is stock, speaker are stock, I could say stock until it loses it's meaning. =p
1. You need the sub
2. a box for the sub
3. an amplifier to power the sub
4. the wires to power the amplifier and connect it to the sub
5. and a way of getting music from the radio to the amplifier
Let's go backwards! Yay!
5. If you are replacing the stock radio with a new CD player, you just need a pair of RCA cables long enough to go from it to the amp. See #4.
4. Go to Walmart! 4 guage wiring kits for cheap. Has everything you need, including RCA cables.
3. This is a toughy. Kicker Comp VRs can be 4 ohm, 8 ohm, or DVC 4 ohm. Worry about that later. Find an amp you like that can do 300 watts RMS (RMS = important) at 8 ohm, 4 ohm, or 2 ohm. JBL 600.1 and Power Acoustik OV2-1200 comes to mind. The forum stickies have a huge list of places that sell and give info on amps. Get back with your findings.
2. Boxes are to a sub what a motherboard is to a processor. Don't skimp on it. If you have average carpentry skills, build one yourself. If not, ask around in the box construction forum. Someone will help you out.
1. Cardomain has them on sale for $99 apiece? I dunno. Hopefully someone can find a better price for you.