Always start with your local College and University courses. Normally they will offer extensive courses in writing techniques. You may consult our file on Graduate Schools or seek help through many private school offerings. Although many regard the writing of comedy as highly specialised we hope that the below links will provide you with proper guidance and valuable information.
THE BBC WRITERS ROOM
A great place to start. You can find considerable information here and it is all free.
Then read this!! Also Free!
Read this! The Seven Laws of Comedy Writing by David Evans
Then read! The Seven Steps to A Successful Spec Script
If your ready to pay a fee for online training then this site is rated the best by Forbes:
Gotham Writers' Workshop