The key to life and humor (which gives so much to life) is to NOT take yourself so seriously.
You're shy because you worry about how you will be viewed, so you hesitate and you play it safe.
You're taking this too seriously. If you can't laugh-how can you make others laugh? Laughter, like yawning and Herpes (sorry Rachel) is contagious. One of the biggest obstacles to a sense of humor is trying to maintain an image. In your case, you can use your own personality as material.
I bet you've been trying to take it outside your realm of expertise, haven't you? The best material is that which you know the best-i.e.-your own life. I'm a skinny white Guy, but I tell people I'm Samoan. When they look at me weird, I explain that I have Vitiligo and a Tapeworm.
Someone who can laugh at themselves can make others laugh. You have all the material you need, if you can only stop worrying so much what others think of you. True humor comes naturally, if you will let it. It is the best perk of being human. Not only will this help you get a sense of humor, you will find that it's very hard to be shy.