There's good advice all over the Writers' Discussion forum, David. You don't need anyone to tell you, it's already there in the old threads for you to find. 
Tips for New Comedy Writers Page 6
Quote: David Smith @ May 27 2013, 8:38 PM BSTWell BCG there is some questions to think about there.
Quote: David Smith @ May 27 2013, 7:14 PM BSTI need a bit of help.I want to write a sketch and I need some good advice.Can anyone help me?
http://www.chortle.co.uk/correspondents/2013/05/13/17841/15_tips_for_new_comedy_writers ?
Why are we so annoying to you, David? Maybe because of your liking of rather nerdish On the Buses? Maybe if you pretended to like Blackadder or The Office...?
Quote: David Smith @ May 27 2013, 8:38 PM BSTWell it seems no one on here will give me advice. Why will someone not give me good advice? Is it because it is against the law for me not to go in to the comedy world? Am I allowed to write? Well BCG there is some questions to think about there.
David, as zooo says the advice is here, you just have to look for it.
Just don't expect to be told how to write a funny sketch.
I presume you are writing anyway (yes, it is allowed) therefore post something in Critique so we can get a feel for your style.
One caveat, please don't write a scene for an On The Buses remake 'cos that IS against the law!