Hey, college students. I'm glad you're back, this town is quiet without you. I'm also glad you've decided to use your bike for transportation. It really is the best way to get around the city. You need some stuff to go with that bike, though.
Please, just get one. I know you're good at riding your bike, but there are a lot of obstacles out there- drivers, cabs, buses, feral pedestrians, cabs, potholes, other cyclists, cabs. It's like Frogger, without the extra lives. Your friends and family would be heartbroken if something happened to your noggin. If that's not enough to convince you- how much are you spending on school? You will only get a return on that investment if you keep your brain intact. Think about it, your brain is getting expensive.
I can keep going... There are 100,00 good reasons to wear a helmet.
You can't buy a new bike for $40, but you can buy a U lock and keep the bike you already have. I know the cable locks are tempting (they are light and cheap), but don't do it (because they're light and cheap). No one can claim that a lock is "theft-proof". You can't promise something like that. But I can say that I had a heart to heart with a guy yesterday about his stolen bike. He had been using a cable lock, and now his bike is gone. He wanted me to be sure and tell everyone to use a u lock. True story.
If you're going to ride at night (or dusk, or dawn), you need to get some. It's the right thing to do. Nobody likes a bike ninja. Without lights people don't know where you are, and if they don't know where you are they have a harder time not running you over, or walking out in front of you. Reflectors aren't enough because pedestrians don't have headlights. You have to watch out for all types of travelers.
There are definitely other objects out there that could make your bike traveling life easier (pump, rack, fenders, rain pants, xtracycle, gloves, clipless pedals... I can keep going...), but start with these three to keep you riding, and safe.