I want to know what I can do when my signature was forged on vehicle registration

If the buyer was aware of the forgery, you may be able to have the transaction rescinded and get the car back.  If the buyer was an innocent purchaser, then you will probably be limited to seeking damages from the forger for the value of your part ownership of the car which you lost.  You could also make a criminal complaint to the police, as forger is a crime as well as a tort.