I have an idea for a script.

I'm in no position to write the whole thing, so I just want to present it as an idea.

I can flesh it out, but I need to know what is the accepted format of presenting an idea to a tv company, and how do I make sure the idea is protected?

I'm aiming the idea at an american audience.

I know you can buy books about this kind of thing, but I simply can't afford it.....

If money's a bit tight, I would strongly recommend getting some books from the library and reading up on the subject.

What kind of idea are you pitching ? Drama ? Comedy ? "Comedy drama" ? One-off TV play/movie ?
Reality show ? Sketch show ? Quiz/panel show ?
They all have different, well defined formats for submission. If you can post some more details, I'll see if there are some suitable web links I can direct you to.

Why are you pitching to USA rather than UK - are you based in USA ? USA guidelines are again totally different from UK ones.

It's an idea fro a comedy/sitcom.

Might work best as an animated series....

It's an idea fro a comedy/sitcom.

Might work best as an animated series....

http://www.bbc.co.uk/writersroom/

You might find some useful tips here.

OK, as far as I know, most production companies aren't that keen on sitcom ideas without an accompanying script. The usual minimum requirement for sitcom is a "treatment" comprising a 1 page overview of the idea, a 2-page step-to-step guide to an episode (ie a scene-by-scene breakdown), and 2 or 3 scenes written out in full. More usually, they will want to see 1 or even 2 completed 30-minute episodes.

Having said that, there are a very few places where you can just pitch ideas. IpsoFacto is one of them, and they take TV material. However they are UK based. I don't know of any equivalents in the US.

I would strongly recommend you read Marc Blake's book How to Be a Sitcom Writer: Secrets from the Inside which is the best one I've seen on the subject. You can get a used copy from Amazon for under 5 quid including postage.

<Added>

Yes, as Lammi says, the BBC website is extremely helpful for aspiring writers. In particular, this page is invaluable:
Writing Sitcom

The Script Factory offers a cheap registration service, which will testify to ownership of an idea at a certain date in case of dispute and anyone who is anxious that their idea could be copied should use this. http://www.scriptfactory.co.uk

I've just read this, if you live in the South West and have a single or series drama series idea:

http://www.devon-cornwall-film.co.uk/2007/01/09/theres-an-idea-tues/

<Added>

that should read 'a single or series drama idea'. Sorry.