How Can I Play My Iphone And My Boyfriends Ipod Through A Car Stereo?

I have a family plan so I've heard that can make the overally monthly cost less but I'm not positive. On my family plan, we have an unlimited texting plan already. Does that texting plan carry over if I get the Iphone? I also want internet use. Any estimate at a monthly cost? ________________ unlock iphone 3g