Writing Treatments can be intimidating but they are the single best way to allow your story to be told with needing your entire script to be read. There are several different ways to write them, each with their own benefit. One of the key things to remember is to ask how the client wants the treatment, what method they prefer.

One of the best sites for writing treatments is here:
http://www.wordplayer.com/columns/wp37.Proper.Treatment.html

It lists various ways to write a treatment, methods on developing treatments, and examples. Realise that most treatments are written after the pitch and before the script, but that's not set in stone. As once you have the script written, it may be easier to write the treatment when you have somebody interested in your script. You can also use the treatment as a way to get actors interested, and as a guide for writing the script.