If you haven't bought your motherboard yet, find and download the manual for the motherboard you want to get. Usually, the motherboard manufacturer QVL (Qualified Vendor List) will be in there. Occasionally, the QVL is a separate download. Both can usually be found on the manufacturer's website.
Since QVLs contain only RAM that the motherboard manufacturer has actually tested (and are often out of date), if you don't like what you see in the QVL, check with the websites of several RAM manufacturers. Most of a RAM or memory finder that should tell you which of their products will work with the board you have chosen (really old or really new boards may not be listed).
If you aren't in a huge hurry to do your build, you might want to wait a monto or so to see what motherboards, etc. may be available for the new Haswell CPUs. They're due out in June and the MOBOs, coolers, etc. will be hot on they're virtual heels.