if i were to write a screenplay and try to pitch it to companies, is it really necessary for me to ahve it copyrighted first, or what? im not too sure on this topic

If you were going to send it or pitch it to companies, you would DEFINITELY want it registered first. There have been numerous cases where studios have literally stolen and used the ideas of other writers.

Register your script with the WGA (writer's guild of america) on their website. It's only $20 and it protects it for 5 years.

you have to remember though: registering your script only provides evidence of when and what was written before the pitching. just because you have a registered script doesn't mean that it's protected.

if someone steals your idea and changes things around enough to be it's own movie (like changing the characters, maybe one of the subplots), it is legally not copying anything.