Where/how would I get a sub/amp installed in my car?

It's always a good idea to have the stereo installed from the place where you buy it from cause these people are usually familiar with their own products and you may be able to negotiate a free install depending on the value of the stereo you purchase.

Go to an accomplished car stereo retailer and make sure you first discuss the install with either the installer or the person you are booking it with. Make sure they understand EXACTLY what you want cause you don't want to have to pay for this twice. Ask around about the place you are thinking about bringing your ride to, the last thing you want it a crappy install job that costs you money.

Im sure Geeksquad only fixes computer problems,its not hard installing amps and subs,but if you get it done by a professional its probably going to cost like $150 maybe a little less

You can always get a SUB and AMP installed at a store from where you buy it. If you mentioned Geek Squad then maybe you are at a Best Buy location?, if so they would be the best choice because then you can get in-store warranty and they are professionally trained.