If you want to write comedy begin small.
Write about the little things, the in between moments, take something that isn't funny and purposely exaggerate it and make it funny.
You don't have to begin your million dollar script or book idea, it can be as simple as a few twists on your thoughts on a small notepad or phone screen.
Watch a TV program and critique it in a funny way.
Rant about something you don't like or that confuses you and write the high points down. Do what you can to capture the moment.
That's it, you're writing. Go for it.