Get the format right.
I once wrote a sitcom episode in my own format before I heard there was 'official' format conventions to use. I then had to redo the whole script which was incredibly tedious and took 5 times as long as writing the scrip from scratch.
I think the BBC writer's room has lots of templates to look at.
BBC writers room is a great resource. Also check out sitcom circle. They arrange groups of like minded warped individuals to get together regularly and discuss writing, critique each others work and maybe even collaborate.
Worth checking out. And good luck!
Never pay to go on a writing course, and never go on a free one - it'll be crap.
Just write something YOU think is funny and send it off to someone who buys or commissions funny stuff. If you have talent, they'll probably spot it even if they reject your offering, and they'll remember you favourably next time you submit something.
And if they do reject your offering but give you some advice, THAT's advice you should probably be heeding.