There are a couple of ways. Each has a different length of time you are protected.
One is to use the Writers Guild of America, West. (WGA) - $20.00 ea
ProtectRite.Com - $18.95 ea
<LINK_TEXT text=“http://www.protectrite.com/default.asp? … ZJLa6tab3r”>http://www.protectrite.com/default.asp?SessID=591587789&AffID=ZJLa6tab3r</LINK_TEXT>
I would choose one of the two and then copyrite my screenplay if you’re in the states.
I don’t know the equivilant if you live overseas.
Copywrite - $30ea