Your current radio will work just fine, as the satellite radio tuner that you buy will broadcast to any free fm radio station. You will have to check the instruction manual of the satellite radio tuner that you buy because adjusting what station frequency it is at ist different for every radio. Once you have chose the frequency it is on, just tune the radio in your car to that station to hear you satellite radio. Pretty easy.
Next question. The tuner you will have to buy normally comes with a car kit which includes everything you need to mount it in your vehicle. (cigarrette power adaper, any docks needed, and an anntenna.) The instructions that it comes with will give you information for best placement areas on you vehicle for mounting your antenna. Tipically, the antenna is magnetic that sticks somewhere on the outside of your car and then you have to route the cable into your car, which can be a difficult task to get the wire routed in and hidden. If your not comfortable doing this yourself, I would highly reccommend you having it profesionally installed.
The only thing left that you will need is a subscription which is available for 12.95 a month, and there will be instuctions for activating your radio include when you buy it.
The only other thing I might tell you is that I would recommend you going with Sirius Satellite radio rather than XM. Sirius plays more the top tracks where XM plays songs that aren't as popular. I have used both and I like Sirius much better.
you can check out sirius's website here. www.sirius.com
Good luck on installing your satellite radio.