Do a search on google next time.
Here's a bunch of tuts:
Richdiesal's tutorial, GREAT FOR BEGINNERS: http://richdiesal.jedioutcastmaps.com/tutorials/
RD is wrong where he says JK2Radiant is better, get GTK Radiant, it is the same program, but GTK is for more games.
Claxon's for more advanced mappers: http://www.cydom.freeserve.co.uk/jk2web/
Map Review, many many tutorials here for everything: http://www.map-review.com/index.php?page=tutorials
Wedge's, good for more advanced mappers too: http://wso.williams.edu/~jshoer/hints.htm
Bubba, yes I know this is for Q3, but JK3 and Q3 have the same engine: http://www.planetquake.com/bubba/tutorial.html