When you're diving, the air in the closed spaces inside your ears is compressed which gives you a "squeezing" feeling. It is necessary to prevent ear damage to "equalise" the pressure.
How to properley equalise your ears
Step 1: Descend until your head is just below the surface.
Step 2: Pinch your nose and firmly but gently breath into it
Step 3: Make sure you control your descent and continue to equalise as you go down
Step 4: If you're still having trouble clearing your ears, try tipping your head to either side as you do this.
Ear Equalisation Tips
Tip 1: If you are experiencing pain in your ears, ascend slowly until the pain is gone and try equalising again.
Tip 2: Never go diving when you are congested or have a cold - even with medicine, if you become congested again underwater you'll get reverse block on the way back up
Tip 3: Equalise early and equalise often! Once you have got the technique down, you should not be experiencing any pain whatsoever!
Tip 4: If you are still having trouble equalising, try the alternate methods of moving your jaw or swallowing.
Tip 5: Take it slowly, a lot of people have trouble equalising after not diving for a while!
Thanks to the Scuba Site