From your diagnosis you have got a ganglion on your hand.
Ganglion is a cyst that is filled with synovial fluid. It usually is finding near joints, as an extension. There is nothing much to be worried about. The cause is just out pouching from joint probably after a trauma or so. It's usually asymptomatic. But, if you are having a problem cosmetically.
You need to first confirm if its ganglion. A dye is injected into the joint, if dye is present in cyst, it's a ganglion. You can consult a surgeon, who will drain out the fluid using a hypodermic needle. Or surgery is also an option, these days arthroscopic surgeries are becoming common.