Get a Hav-a-Hart trap in the appropriate size for the type of rodent you wish to relocate. I'm not sure of the best source, but a web search will likely bring many good options. Rats and mice love peanut butter, but if you put that in the trap, make sure to mix it with enough water (or mashed banana) to prevent them from choking. I also suggest putting an extra dish of water in the trap in case they're in there for awhile before being discovered and relocated. Set them loose to a safe rural location as far away as realistically possible. They are very smart, and will otherwise likely come right back! Of course, if there is more than one, it's best to relocate them all in the same location so they can find each other once released. And once they're relocated, do a thorough inspection to see how they got in. Then repair any holes, seal all entry points into your home, and clean up all food temptations so they don't come back thinking they've been invited! There are actually some humane companies that will come out and relocate the rodents and then rat-proof the building, but they're not all that common (and it's important to confirm that they truly do relocate them humanely).

If you don't have companion animals who happen to be rodents, then it may be helpful to also get an ultrasonic repeller, assuming there's a way for the rats to escape. I'm not sure how well they really work, but here's a link to one that may help:

http://www.whateverworks.com/itemdy00.asp?c=&T1=K7816&GEN1=Pest+Control&SKW=Pest&PageNo=1

Also, there's an all natural essential oil product called Mouse Away that I recommend. Even though it's called Mouse Away, it also works for rats. (However, the humane traps they sell are too small for rats). Here's the link:

http://dreamingearth.com/ushop/index.cgi?=&task=show&cat=Natural+Pest+Control

Good luck! And may the rats and mice run free! :)