A good way to start riding a unicycle, is a pole. Any pole will do. I find that a basketball pole is sufficient. Grab tight to the pole, take the foot furthest from the pole, and pull yourself up! At first you will hug that pole like your life depended on it. Don't worry, this is normal, you'll slowly let loose after awhile.
Remember, don't cling to the pole. You eventually will need to let go, and pedal forward. If you need to fall.... just jump off! You won't get banged up on a unicycle.
The trick, is to keep with it. Only one out of twenty people who get a unicycle, learn to ride it.
After you've conquered going strait... You may want to try unicycle hockey!