well under my doors theres rust that had built up...and i wanna get rid of it.....but i dont rilly kkno how to..so can someone help me here?

depends on where its out

I'd try to sand it all off. Then use bondo and fill in the area missing. Paint.

go to an auto store and buy some stuff called Navy Jelly, gets rust off then sand it down till it smooth and the fill it with a plastic filler, or just some spot putty if its not that big

Just remember, you have to either seal in the rust with something made to do it, or get it all off (sanding/grinding), fill the area, and paint. You can't just paint over it, it'll come back. And if you sand and fill it, be sure to paint the area as soon as the filler is dry. Regular bodyfiller is not waterproof, so it needs a coat of paint to protect it from rusting again.

Ryan