I want to post some puzzles and games, but I don't know how get the FEN string.

 Can anyone help?

Most software packages can give you the FEN for any position. Check out Chesspad. Very usefull and very free!

Or.... do it the hard way and code the FEN yourself. The FEN notation is well described at, say, Wikipedia

http://www.google.com/search?source=ig&hl=en&rlz=&q=chesspad

:)