Know the joke. Period. If you're not sure go over it in your head - all the way to the punchline - before you start.
Tell the joke from a standing position. Witicisms and puns are fine while seated, but jokes work best when you're standing up.
Be animated. Smile and move your arms about. Look happy to be there.
If you're nervous, pick just one person in the audience and tell the joke directly to him/her.
Speak in a firm, confident voice. It's not funny if no one hears it.
Enjoy yourself. Sometimes it's even funnier if things don't go as planned.
OK, here's a joke to start with: A guy walks into a bar with a duck on his head. The bartender says "Can I help you?" and the duck says "Yeah, can you get this guy off my a**?"