The most important thing is to make sure that the suit fits properly. Very difficult to do with off-the-rack suits. I highly recommend that you do not buy a suit anywhere there isn't a tailor in-house or at least someone to measure you properly. Cut is much less important if the suit fits your body.
Avoid black suits. They can look cheap easily, or too formal, or just uncreative.
Don't overlook the details like belt, shoes, socks, tie, and especially the shirt. They should all be the same level of "formality." Don't wear a casual belt with the suit, or non-dress shoes or socks. Make sure your shirt is nice, not some oxford cloth, button-down collar, throw-in-the-dryer-to-get-the-wrinkles-out thing. Make sure your shoes are polished, your tie is straight, nothing is wrinkled (beyond one or two creases from packing).
Are you in SoCal? Do/did you go to CSUN? If so, there is a chain of stores called 3 Day Suit Broker in SoCal who are only open on weekends but have a good selection of inexpensive suits and an on-site tailor. If you're on a budget (who isn't?) you can get the whole ensemble there for a good price.