This is a question I get asked a lot. You can send it in to script competitions. No harm in that. (I write about that here) You can send it nebulous corporations who have a public service remit and will genuinely read it eventually. Better than leaving it in a drawer. I shouldn't bother sending it to an agent (unless it's a movie script or novel).
But for sitcom, I give the same answer every time. Send it to a producer who makes programmes you like - and whom you think will 'get' what you're trying to do. Print it out, put it in an envelope, spell their name correctly, write a polite covering letter that doesn't make you look like a nutjob, a stalker or a precocious 12-year-old (even if you are, hide this fact). Then wait.
If it's really good, they'll call you. Really and truly. Most scripts aren't any good, including those written by experienced professionals. So if you've written something that isn't even broadcastable, but shows promise and talent, they'll call, email and contact you somehow eventually.
Bear in mind they have work do, a job in hand and it doesn't really involve you - but they need shows to produce, and every time they open and envelope, they fear the worst, but hope for the best.
If you don't hear back ever, and you've sent it three different producers, maybe, just maybe, the script isn't as good as you thought it was. In which case, do what all decent writers do: do it again. Rewrite, edit, change, delete, type, scream, delete, type, read, simmer, pause, read again, edit then send. If you're not prepared for any of the above, may I recommend another job?
Any producers on the receiving end, please feel free to confirm or deny any of the above, but that's my experience and recommendation.