You may be tempted to crack straight on with a sitcom, but start small. Containing an idea in a two-minute sketch will teach you about structure, establishing characters and how to write pithy, economical dialogue.

It is easy to put on a sketch show at your college, pub or on the internet. A producer will be happier to read a page or two rather than a whole script and there are radio and TV shows which are looking for shorter sketch material, which means you have a much better chance of selling your work.

I started my professional career writing topical sketches for the now defunct Radio 4 show Weekending. I actually pretty much loathed the programme, as it was rather formulaic and rarely biting. Yet I stayed for a year, serving an apprenticeship that taught me many skills: from the mundane business of how to format a script (for this and further advice see bbc.co.uk/writersroom) to technical tricks such as how to avoid clunky exposition like:

FX: Knock on door

MAN: You asked to see me Prime Minister!

This opening establishes location and characters artlessly. You need to look for more subtle ways to inform the listener or you will lose their interest and respect. Don't treat them like they're stupid.

I soon learned that even though we were paid by the minute, that it was foolish to write 5 minute sketches. The show was only 25 minutes, so longer skits would be binned, while lightening gags might fill a gap. It was economical to be economical.

Now I prefer to stretch an idea as far as it will go, then a little further. If you can learn to write a blistering 60-second skit with four laughs, a beginning, a middle and an end, then everything else will be easy.

While Weekending is no more, there are plenty of sketch shows on radio and TV that invite outside contributions. If there are lots of writers' names in the credits, write a couple of sketches in an appropriate style (even if it's not your particular sense of humour), send them to the producer and you will probably get feedback.

Or you can set up your own sketch group and take a show to the Edinburgh Fringe or film it for YouTube. Try to make your own material as original as possible. When Stewart Lee and I began writing together at university, we set rules about things we wouldn't write about: celebrities, parodies of TV shows, political satire, all of which were in vogue. By limiting ourselves we came up with a lot of unusual ideas and created our own voice.

Sketch writing tips

· Keep an eye out for interesting real life characters. My driving instructor seemed overly critical of my inability to drive, given that that was the reason I was employing him, so I wrote a sketch about an instructor who berates his pupils for being non-driving idiots.

· Don't start with a catchphrase. It will seem forced and probably end up with you creating a one joke persona. Create the character, write some sketches and a catchphrase might present itself. Look at Al Murray the Pub Landlord. It's a multi-layered persona and the catchphrases "I was never confused", "rules is rules" and "glass of white wine for the lady" come out of the character rather than vice versa.

· Starting with a simple premise and exploring the consequences can be better than trying to conceive something outlandish. Monty Python's dead parrot sketch begins with the premise of a pet shop owner selling a customer a deceased bird. The genius is in the execution.

Exercises: Character comedy

Watch a whole morning of daytime telly. Look out for an interesting character and then try and write a sketch about them. Don't try to parody the shows you have watched, just try to find a persona and then put them into a real life situation.

Many of the Little Britain characters were created this way.

Mitchell and Webb on writing sketches

Make sure you have an idea before you start. It's no use sitting in front of a blank screen saying "right, it could be anything." "Anything" isn't a brief, it's a mental wilderness. You need to decide what you're going to write before you write it, and this is best done away from the winking cursor.

A sketch needs a premise, a core funny idea that is its reason to exist. As soon as a sketch begins, the audience looks for this premise and it needs to be apparent. Presenting a character? Make sure the funny thing about them is expressed early. Taking the piss out of some element of modern life? Present it at the beginning and quickly undermine it.

You need the element of surprise in comedy but, before that, you need to make people comfortable with where you are. There need to be, to quote the protesting philosophers from the Hitchhiker's Guide to the Galaxy, "rigidly defined areas of doubt and uncertainty". So establish the setting first, make it clear why it's funny, throw in a surprise and get out. Ideally the last joke, or punchline, should be the best but the sad fact is there are more premises than punchlines. It's a great argument against intelligent design.

Sketch comedy doesn't benefit from the audience's loyalty to characters, it's only as funny as its last joke. But its advantage is that it can embrace any setting, subject or situation. Use these strengths by having lots of short and contrasting items. That way, if the audience doesn't like one sketch, you soon get the chance to win them over with something else.

· David Mitchell and Robert Webb are the creators of TV series That Mitchell and Webb Look.