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.
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.
now i suck with the system stuff but i will try to explain what happened to my car. i was in your shoes and just wanted a little more base. i went to a local shop and had a 12"JBL sub in a ported box with a TMI 250 watt amp. i to used the stock radio. when i had it installed the box that the install shop had to use, because there are no rca hook ups behind the stock radio, made my car do some weird stuff. the windshield wipers did what they wanted to. the shop did not know what was going on. since this was the first 07 they installed anything on they called around. it turns out that the "modulator" (i think that is what the box was called) was emitting a signal that would make the wipers turn on. i know it sounds weird but i swear on my mothers eyes that is what happened. i think the took the signal for the speakers and stepped up to a stinger unit. all in all the shop was great and fixed my problem. i am now very happy.