Most companies ask for submission via agents and I'm not sure where to start re: knowing who's legit / decent. Could you recommend an agent?? I'm desperate to make it!!
Your best bet would be to go through the Artists' and Writers' Yearbook 2007/08 (or buy last year's copy on eBay) where there's a full list of agencies, and who to contact. Approach those you think are the best fit for you (they may have clients that you like/admire for example) and take it from there. Most agencies have websites so you can check out their details on a more detailed and informed level.
The big agencies are companies like Curtis Brown, PFD, The Agency, Independent Talent Group (formerly ICM), while there are many reputable agencies like Linda Seifert, Sayle Screen, Micheline Steinberg etc.
If you're desperate to make it, then start getting in the know. Read 'Broadcast' magazine or 'Screen International'. Or both. Start attending seminars/workshops. Write down the production companies/producers/writers/directors of your favourite shows. Write to them. Try to meet them. Anything. Get ahead. A lot of what you do will come down to your writing ability but that doesn't mean that if you write a great script, your career will begin. There's a lot of luck and networking involved as well as total commitment and focus from yourself to making your career happen. It's difficult, and at times soul destroying, but it's not impossible.