If I order a wheel and tire combo online and they come mounted and balanced, what do I do when I get them? Can I just fill em up with air and install them on my car? Or do I still have to take it to a shop to get it installed?

 

You don't even have to add air (you should check the air pressure though).

Just install them with the proper lugs and hubcentric rings and you should be good to go.

 

what are hubcentric rings?

 

there little plastic rings that go around the hubs you'll see a big bolt in the center of the studs it goes around the outside of that.