What do I need to know about motorcycle helmets?

I'm signing up for a rider's safety course to get my motorcycle license next month. I know nothing about motorcycles or helmets other than they get chicks, hence why I'm doing it LOL.

I need to pick a helmet before class starts. What do you recommend I look for? I was on eBay. Some of them are around $160-200 while some are around $50. That doesn't make sense to me. Are the cheap ones unsafe?

What color helmet should I go for? I want to get a sport bike but I have no idea what color its going to be.