Consider this, writing standup comedy is not really writing in a true sense or at least it's a different form of writing. Perhaps it's more related to creating a scene with words? Whatever it is you are in fact simply capturing an emotional moment and putting words to it. As few words as possible of course...never forgetting that these words should have an unexpected twisted jolt at the end that will cause an audience to react (hopefully in laughter).
I often tell my dojo that standup comedy is related to gossiping and ranting. Just the emotional facts ma'am.
Standup is NOT about being perfect. The pefection comes when the audience is rolling with laughter and amazement of your personal twisted thinking.
If you're stuck for writing something, simply go on a rant about something. Dont' even try to be funny, just go off, capture it and rewrite it to jokes and bits. It can be done easier than you think. More on that later.
Now go be funny and make somebody smile at least!