the OP in
this thread has tons of useful tips, but is geared more towards "The Uniform", classic buisness style. Meaning, the best way to not screw up ANY position. You'd be surprised how some seriously some people take an Interview suit.
If you're just going to get one suit, i'd recommend Charcoal Black. It's pimp, it works fine for an interview as much as a night out on the town, and it's different than the standard navy or black. Also, make sure it's not too high of a thread count, and NOT double breasted. You're not out to look like the Teflon Don. Keep it either two button or three button and around super 100's-130's (this is the thread count. Higher the thread count the 'shinier' the suit, some Italian suits will go upwards of 180, and this not only makes the suit wrinkle easier but also makes it look like you wanna cook eggs off your chest).
If you're just going to get one suit, either get no pinstripes, or get very subtle chalk-stripes. If you plan on getting others, I like at least one suits that has very distinct pinstripes, as it looks very nice against quality worsted wool.