First, try getting a can of compressed air. Stick the nozzle under the key with the can upright and give 'er a few blasts. If there's anything stuck under there (crumbs), that should get it loose.
If that doesn't work, try removing the problem keys, cleaning everything thoroughly, and putting them back on. I'm sure you can Google instructions on how to do it.
http://forums.mactalk.com.au/showthread.php?t=8714 has some steps, but the pictures aren't there anymore.
http://docs.info.apple.com/article.html?artnum=58390 also covers some things like software issues.
Good luck!