You NEED a pump, a spare tube, SOME sort of inflator (CO2, frame pump, etc. I recc'd CO2), and tire levers.
You basically NEED bottle cages and two water bottles, and bibshorts.
You should have sunglasses, real biking jerseys, and cycling fingerless gloves.
You don't need a bell, a light (Well, you decide that one, most don't,) fenders, etc.
I recommended the Sugoi RS bibshorts. Buy cheap, buy twice.
I'm positive I'm forgetting some stuff, but that's a good start.