It's pretty tough, but it can be done. I'd suggest starting off with either:

1) A simple premise/question, e.g. "Why can't cats fly?", or "A horse walked into a train station". (Yes, animals are generally good fodder)

2) The punchline; "It was only Monday!", or "She didn't see him coming!".

3) A word which can also sound rude, like 'Bumblebee' or 'Cockney'.

Then it's just a case of thinking, and thinking, and... yeah, actually, it's really tough. Good luck with that.