If you take time to investigate someone who appears to be "naturally" funny, you will find that they developed their ability to be funny through exposure and experience. You can learn to take a shortcut to developing that skill by following these few simple steps.
Have an idea but don't know how to get to the comedy? Here's 5 killer strategies to help you write comedy on a simple idea or word. It's called 911 for your Jokes by comedy writer Jerry Corley.
When you're starting out, I cannot emphasize the importance of building that core act. You should do it constantly, revise, refine and polish. Add act-outs, tags and toppers. Until it crushes. Worrying about your friends hearing the same jokes is counter-productive to you really developing and polishing your act. Not to mention that it can have a cascading negative impact on your development.