That's easy-- Think of what you want your topic to be, write out a quick outline:
Such as: Skit opens with two dogs--
scene: walking down the road:
Dialog: Talking about who is going to go to the pound first--
ARGUMENT BREAKS OUT BETWEEN THEM--
Punchline: They both get hit by a car and killed--
fade out to black.
((Dark Humor)) But it is clean--
Best tip that I can give you is this when it comes to writing a scene for a story or a skit: Think of a scene that you have seen in a movie--and figure out what makes that scene work. Don't get to crazy with discribing things such as where they are or what's going on around them...but don't forget about it either. It is best to paint the picture for a place or an activity as soon as possible in your story...then leave it alone mostly...the reader already knows where the characters are by now.
And if you want the BEST way to make dialog for anything work...talk it out alone in your car on the way home from work...as if you are the people in the story itself. And do the scene a few times in different situations in your head...then write it out. By now you should know what you thought was funny and what wasn't.
When it comes to writing something clean yet funny...try using the old school approach...mild slapstick violence...like Bugs Bunny, Tom and Jerry, ichy and scratchy...I hope this helps.