Legal Question in Real Estate Law in California

There is a tort called "conversion" which is committed when someone wrongfully asserts possession or similar rights against the property of another. The remedy for conversion is money damages equivalent to the value of the property detained. You might look up "tort conversion" on Google, and review some examples. Then get a copy of Nolo Press paperback book, "Everbody's Guide to Small Claims Court in California" by attorney Ralph Warner and read up on small claims procedures. At 20, you are entitled to possession and control of your own property, if it is really yours, and your father doesn't have a lien on it. Merely filing a suit against him may result in him being more reasonable. However, I haven't heard his side of the story so I can't comment on any defenses he may have or his reasoning for denying you possession.