You could start by just replacing the stock speakers with better ones. I would hit up best buy and circuit city and make them demo them for you to see which you like best, then buy them online where they are cheaper.
If you want to add an actuall sub you are going to have to pull out the stock unit to see if there are audio out ports for it otherwise you are going to have to replace the head unit too.
Just grabbin some new speakers would be your cheapest option, and you wouldn't have to take apart alot of your car to wire the sub/headunit.
The wiring is very simple and I can help you out if you get as far as doing it yourself but if you want somebody else to do it find a good repped audio shop.