You want as fast as possible, f/2.8 at the minimum. The Rokinon 14mm has already been recommended and as cheap as it is you can't really go wrong, so I'll second that.
Also, a few tips that I wish someone had told me before I wasted hours shooting in the wrong places/wrong time of the month:
1. Get away from major cities, as far away as possible (see this map: http://www.blue-marble.de/nightlights/2012
), this makes all the difference in the world. I don't care how dark it is in your back yard on a dark night (unless you live in middle of nowhere Iowa or something), if you live within any range of a even moderate sized city it's going to turn out like crap. If you live in the western half of the country this will be much easier for you than the east.
2. GO DURING A NEW MOON. This has a HUGE impact on how good your images will turn out, I mean massive, cant stress it enough. Consult this page: http://www.moonconnection.com/moon_phases_calendar.phtml
3. Download Stellarium to find out where interesting areas of the sky will be. To find the really amazing part of the Milky Way you see in tons of pictures look for Saggitarius.
4. Get a rock solid tripod, turn on mirror lockup, turn off in camera noise reduction, turn on Silent shooting if you have a 5D3, set your WB to 4000K, ISO 1600-6400 (depending on conditions/what you're going for), open the aperture up, remove the camera strap (wind blowing can create vibrations), and tape over the viewfinder. As for exposure time the rule of thumb is that whatever your focal length is just divide 600 by it and thats the max exposure you can shoot without getting trails. So if you get a 14mm, 600/14mm = 42.8 so you can do a 42 second single exposure before the stars begin to show as an oblong shape.
Honestly the biggest thing you can do for yourself is getting to the right location during a new moon, I was absolutely floored the first time I saw the results shooting out in the Texas desert on the night of a new moon. Suddenly I realized why all the pictures I had taken in Atlanta were shite, it wasn't because I sucked it was just the conditions. Anyways, godspeed and good luck.