Wear a Helmet
The most important thing you can do to stay safe on a motorcycle is wear a helmet. Helmets protect your head, and head injuries are the leading cause of death for motorcycle riders. Your brain just wasn't meant to be exposed.
Of course, wearing a helmet isn't enough. You need to wear the right helmet. Lots of places sell novelty helmets that look good but don't protect you. Look for a Department of Transportation-approved helmet. Those helmets have been tested and provide a minimum standard of protection.
Make sure your helmet fits properly. It shouldn't be too tight or too loose. It shouldn't obstruct your vision, but it should cover most of your head. For more protection, opt for a full-face helmet with protection all the way around. A simple windshield can at least protect your face from bugs, rocks and rain, but a full-face helmet will have a piece that goes around your chin, so your face won't make contact with the road.
Even if you ignore every other motorcycle safety tip, wear a helmet.