Look for an instructor who's into respect and discipline, but still has plenty of patience. The class area should have lots of space and a smooth, flat floor with padding. The fewer students the better—more attention for you!
Wear all the right gear. Warm up and stretch so you're loose and ready to go! You need good instruction before launching into any moves. And when you do learn the moves, remember your limits. For example, white belt students shouldn't spar (practice fight).
When you are ready for matches, you've gotta have an instructor around to regulate. Some martial artists use special weapons (like swords), but it's almost a sure thing that you'll get hurt with them unless you're totally advanced... so, no weapons. During your match, make sure that your partner knows when you're ready to stop. If you let your guard down, your partner may think it's a good chance to take you down!