Make sure your bike has a white reflector on front, and a red reflector on back. It is also helpful to have reflectors or reflective tape on your wheels, and on your pedals (any part of the bike that moves a lot when you ride is a good candidate for reflective material — the movement helps drivers see you.
If you ride at night, install a white headlight, and a red taillight on your bike. Although you get what you pay for, a cheap light is better than none, but isn’t your safety worth top dollar?
Get a pump, patch kit, and tire levers to fix a flat. Make sure you know how to do this before you need to!
Consider installing a rack and buying panniers to carry your books and other items. Moving the load off of your back and onto a bike rack makes riding more comfortable and more stable as the center of gravity is lowered. Make sure the panniers you are buying are easy to mount and remove from your bike, as you will need to do this often. Rain covers are a must in most areas.
Consider installing fenders on your bike. They nearly eliminate the spray your wheels make on wet roads, keeping you drier, cleaner, and more comfortable. Full fenders are more effective than partial ones as they cover more of the wheel.
Consider a glasses, helmet or bike mounted mirror to help you see traffic. Cyclists often have a tendency to swerve when they look behind them!