Squats...
Squatting is great and one of my favorite body weight exercises to do. Keep your chest up, stick your butte out as you descend, keep your knees apart and parallel, and drop down until your thighs are parallel with the floor. Repeat until you can do no more! Yeah, feel the burn!
Poor Technique: Knees going inward
Poor squatting technique is evidenced by knees going inward or adducting. Keep them parallel!
One-legged squats are excellent strength and balance builders. Keep your hips over your feet, descend by bending your weight bearing knee. Have the other leg straight out behind you and your hands/arms out in front of you for balance