The first rule of comedy is that there are no rules. Watch TV - or better yet, live performance - and you'll soon see that one man's meat is another woman's bullshit. A lovely image, I'm sure you'll agree. For every 'just discovered' genius there are a 1,000,001 pond life scrabbling about for the odd morsel. I say that with some fondness, rarely coming up for air as I do. I'm no expert but here's my two penn'orth.
If you're not already on the telly, radio or premium comedy circuit, you might be tempted to put something on the web for free, just to be discovered. Unless your idea is innovative and sharp, I suggest you fight that temptation, like the urge to fart on a full stomach. Trust me, it'll probably end in tears and / or laundry. If your concept is good enough, approach a production company.
There are non-commisioned niches though - I've had a few gags on radio, some material for live performance, a little bit for stand-up and a semi-infamous badge in the USA. I've also attempted 3 sitcom scripts, a humorous advertising campaign for sexual health and approached several sketch shows on radio and TV. More about those some other time, in my list of heroic failures.
Comedy is really sexy at the moment. If you can get an 'in' to anything, burrow deep and hang on for dear life (that wasn't the sexual health material, by the way). Any contacts you make should be cultivated and maintained. I once emailed a producer of a Channel 5 sketch show on spec and that's how I eventually got some paid gags on radio. The old adage about 'who you know' holds true.
It also helps to establish a writing voice. My own brand of comedy writing tends to fall into three distinct categories: 1) Wordplay, 2) Coarseness and 3) Cynicism. And sometimes I try to combine them.
A wise soul (Bill Hicks, I think) once said that comedy is about finding similarities between different concepts and differences between similar ones. I think Comedy is also a theatre of cruelty. Even when someone isn't the butt of the joke directly, the audience is seduced by the notion that we know what's coming or that we're smart enough to see an association or veiled reference. It's also been suggested that comedy is tragedy + time. Tell that to the man crushed by a grandfather clock.