Begin learning proper technique by sitting on a bench or chair. The key is to think of sitting back and not down. This will keep you from bending from the hips first instead of from the knees. Once you've mastered the chair or bench (using different heights) it's time for free squats or air squats! Common cues to help you with your air squats are: “chest up”, “sit back” “heels down” and “knees out”.
Here's one of my favorite videos from Crossfit...2 minutes of straight squats with lots of great cues scrolling across the bottom. Check it out and get your squat on! :)