Ooh, some more advice:
Get a pilot made, and let it go viral online. The internet is an animator's best friend in this age, why limit your cartoon to just a few people when you can let everyone in the whole world see it. Once a pilot becomes an internet phenomenon networks, will be lining up to pick up the rights to make it a full on show, because they see for themselves that the show is a hit.
Making a story bible will help too. With an outline, a season's worth of ideas, character designs and settings, etc. Actually, you should do both, have the pilot up on youtube, and have a website for the story bible.
Also, keep in mind that you don't have to go to a network to get your show picked up. Going to any animation studio who's interested in producing animated series would be good too. They can even produce a pilot for you. The downside is that there's no guarantee that your show will get picked up on television, or at least air in the U.S. Your show might end up getting picked up by a channel in Europe instead.
Also, when Cartoon Network came to my art college, they told us that just pitching a simple sentence will get them interested. It's all a matter of it being a good idea and getting in contact with them.