It's probably too late, since you posted that over three weeks ago. You can't fix your own cavity, but, if there's a visible hole (as from an old filling that came out), you can buy a dental repair kit at the drugstore. It contains oil of cloves (old toothache remedy), a couple of Q-tips, and some cement (either mixed and sealed up) or dry, to mix yourself. This is just a stop-gap. It may help with pain or sensitivity by keeping the air out of the cavity until you get a dentist to fill the tooth permanently.
I answered this question seeriously, and am assuming you are serious. Lorryann