Sorry this answer took so long:
Writing funny, original dialogue should come naturally. If you find that it does not, then funny may not be your style. There are no classes or books on being funny or finding your inner stand-up comic. The only choice you will have, would be to find a friend who is the 'class clown' type and call on him to help. No harm in having more than one head to write a story. If you feel that you must read something to inspire your comedy writing then I would suggest, D. B. Gilles.(Dbgillescript@aol.com.)
Writing funny is different than saying or doing funny things. In life, most people lose their focus when telling a joke or a funny story. So they end up losing their audience. Since there are different types of funny out there in life, your first goal will be to find out which one appeals to you the most. The audience for a comedy screenplay is much tougher to please and much more unforgiving than any other play. So goal number two will be making sure that you keep them laughing from the first page on.
You will need to make sure that your story is absorbing enough. A great story goes a long way. Even if it doesn't have as many laughs as you would like, it will still have the reader engrossed enough to finish and you want the reader (especially an agent or publisher) to read the story to it's end.
I would also suggest reading the five secrets for improving your comedy writing by Fred Rubin: http://www.scriptfrenzy.org/node/413100
Good luck and I hope that by now...you will have at least started your screenplay! ^_^
