Starting college soon huh? I have a buddy of mine that paid his way through college doing t-shirts. His target market was fraternities and sororities...they love buying that kind of stuff.
But you're looking to far in the future, investing in an expensive heat press or screen printing machine is going to need a larger business plan than "I want to see how it goes." These machines arent cheap, so I advise you to first learn the methods of printing, draw out your designs, and lay out a good business plan.
I learned everything I needed to start by just reading past forums, so if you are serious about it, I would refer to all of the past threads....it may be tedious, but hey...if it were easy, then it'd be no fun.
Everything you are looking for has been covered in these forums.