Lights are absolutely essential for night-time riding. Most cyclists who get killed are hit at night, and most don't have lights. The only light required by law is a front headlight, but you should also have a rear red flasher even though the law doesn't require it. This is a cheap way to keep you from being invisible. Get these at any bike shop, although sometimes they can be had at places like HEB, Wal-Mart, and Academy.
Red flashers are cheap and effective.
This is a no-brainer. With headlights , you have a choice. You can get a light similar to red flashers, but with white, yellow, or green light. The advantage is that it's cheap and the batteries last forever. It's bright enough to get you noticed in some situations, but it's not as noticable as a white headlight.