Tire and Rim Dimensions

I've been driving my Maxima around for about 2 years now and never really thought about doing anything to it until now. I want to put on some Low profile tires and Some cool rims, But I have no clue what the Dimensions are... I know when I go to buy rims I'll be asked what size I want/need etc... Can anyone tell me what I'll need to know before buying? Thanks a lot.