Depends on what is wrong with it. If just rusty a 30 minute, 50/50 white vinegar and water bath will remove most of it. You might have to scrub a little but you can repeat the bath 30 minutes at a time. Any longer and it will start eating into the metal. I know one guy who forgot about it and left a cast iron skillet in for a few months. You could crumble the skillet with your fingers.
After any rust is removed polish and sharpen, coat with a little food grade mineral oil.