Dress as nicely and professional as you can afford. I strongly recommend wearing a suit. Next best is sport coat and tie followed by slacks, dress shirt, and tie. Most will not consciously hold it against you if you don't wear a suit as long as you dress nicely, but your competition may gain an edge in perception if they appear more professional. Often perception is unconscious and the interviewers don't realize they are reacting to appearance. And your confidence can suffer if you are the only orange in the apple barrel.
I worked my way through graduate school selling high-end men's clothing to businessmen, young professionals, and students preparing for interviews. Here is some advice:
Suit: Dark. Preferably 100% wool, but a good wool/polyester blend is ok. Solid navy blue or charcoal gray. Avoid pin stripes or patterns unless you know what you are doing. Otherwise you risk looking like a gangster if you get the stripes or pattern wrong. Fit - slim to full depending on your build. It should drape comfortably without looking like it was painted on or like a parachute.
Shirt: Solid white or light blue with a point collar. The spread of the collar should be moderate - not extremely wide or narrow. Button-down is acceptable, but is considered less formal than point collar. Button cuffs, no french cuffs.
Tie: Silk, moderate width. Stick with ties that are primarily some shade of red or dark blue with other colors as accents. Pattern - Rep, Club, Paisley, Solid. Can have a little flair, but nothing too wild.
Shoes: Black, leather, well shined. Oxford or Cap toe.
Belts: Black leather with modest silver or gold buckle. Moderate width 1" - 1 1/4"
Socks: Over the calf. Dark - navy, black.
Go to a men's store like Brooks Brothers and ask them to show you reasonably priced clothing appropriate for an interview. Many have clothing lines geared toward the price range of a younger person. If what you see is out of your price range, you will at least get an idea of what to look for in a department or discount store.