Here are some tips...
Try to keep your lungs filled with air. Don't hold your breath, but don't exhale all the air in your lungs while flailing your arms and legs. You'll be amazed at how bouyant you'll become with a good volume of air in your lungs.
To tread water:
1) Move hands/lower arms in a horizontal, circular motion. Hands should move in opposite directions.
2) Egg beater kick: Largely accomplished by the lower portion of your leg (below knee). You will simply move your lower leg in a medium-sized circle, with a good bend (nearly 90 degrees) in your knees. Your lower legs will point towards the bottom of the pool and move opposite directions (as with the hands).
NOTE: Move the right arm and right lower-leg in the same direction - say counter-clockwise. Move the left arm/leg in a clockwise direction. The motions of your legs will be off by 270 degrees of rotation (as will the hands be offset from one another by 270 degrees). Your hands will move at about double the rate of your legs.
You can practice the motions necessary to tread water while sitting in a chair. First, focus on the legs. Sit upright in a chair, knees bent at 90 degrees. Pick your feet slightly off the ground and practice the egg beater kick. When proficient, add in the hands/arms movements. Tada!! You can now tread air/water.
EXPERT TIP...If you ever fall from a good height into water and you cannot tell its depth, begin treading water powerfully the instant you hit the surface of the water. This will DRASTICALLY reduce your momentum.