I am looking for a website with worthwhile advice on writing funny dialogue. Can anyone please suggest one?
Thanks
Gary 
Online dialogue resource
Grammar and layout can be worked on but looking for a site that shows you how to actually write funny would only leave with the same sense of humour as the person who wrote the advice.
The best advice I can offer is to visualise the funny situation you are thinking of and then write it like you where there and overheard it.
The only really worthwhile advice is to get the story right, people it with conflicted characters and let them speak.
Dialogue isn't a substitute for story.
And it's NOT about jokes.
And to be honest, if you can't write funny, don't write funny.
There are all sorts of genres you can write in - try them all.
But the basic rule applies - convincing dialogue is a natural bi-product of character and situation.