A good lubricant like Pro Link will take off superficial rust, or you could use solvent. Wiope it off with a clean rag and re-lube. If the chain is really rusty, that is, if Pro link doesn't get it off, change it.

The best way to prevent rust on the chain is a fender and mud flap. Well, not eaxactly, the best is a fully enclosed chain case, but that would only work if you have an internally geared hub or a single speed.

If you ride in the rain wipe off the chain and lube it right after you finish riding.