The others might be right, you might need new blades or something. What I would do first is to: A. Be really sure that the blades are nice and clean B. Purchase a fan blade balancing kit for a couple bucks and see if this quick fix takes care of your problem. These are just small weights with adhesive that stick to the top of the blades to balance the set. do a google search for Angelo Fan Blade Balancing Kit or find it, or similar at a local store.
Obviously, if there is something significantly wrong then this won't fix it.