Hon!!! First things first:
Rest your leg. Right away. Stop what you are doing, and put your leg up.
Ice it -- this reduces the swelling. Use a bag of frozen peas, but protect your skin with a thin piece of fabric, like a tea towel.
Compress it -- use a tensor bandage around it to give it support.
Elevate it -- get the blood from pooling in the are.
DON'T keep on it. Just stop, and do the first aid treatment. You can take Motrin for pain and reduction of swelling, too, to make you more comfortable.
If you can't put weight on it, or feel that it hurts "too much" (if you know what I mean), get to an emergency room. Also, if you have weird bruising around your toes or anywheres on your foot/ankle, get it checked out because it might be a fracture.