Where can I get suit measurements?

Alright, so Ive never actually worn a suit. I'm a college student and I usually just wear a dress shirt, slacks and tie for whenever I have some kind of interview. But its about time I get atleast ONE suit (jacket, pants). Firstly, can anyone explain the basics of buying a suit....does a suit always come with pants or..separately? Where can I get measured? Does stores like macys do it for free? And when I buy a suit...what should be looking out for? As in..what are the features of a GOOD suit?