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.