You should already have the wires since you have the deck/amp/sub setup already.
You would only need to drop in the new sub, so to speak - box w/ sub... and make sure the ohms/ratings are a good match... otherwise it will sound like *** - or blow your sub or your amp or something.
For a good audio basics forums (as well as advanced stuff), check out
http://www.the12volt.com, read up on sound systems on circuit city/best buy site, etc - they cover the basics there. You can also go to a best buy/circuit city and talk to the sales ppl/tech there - just don't buy anything from them, but you can usually get some basic info.