Create a synopsis; the more detailed the better. While it is certainly possible for another person to come up with a similar concept, it becomes less likely that she can come up with a concept with your character's name or background or the setting of your show, or any other specific details.
Keep your idea closely guarded. Just because you're excited about your idea, don't go sharing it with co-workers, neighbors or even friends and family until you have performed the steps necessary to protect your idea as your intellectual property. It isn't a matter of distrust; it's just good business sense. You have a right to protect your work.
Copyright your concept. Copyrighting your concept is a near airtight way to insure that your work can't be used by someone else for their own gain. You can also send a copy to your lawyer. (See Resources section.)
Send a copy to yourself. This is a something writers have been doing for years. It is not the most concrete way to protect your idea but it can prove useful if you ever have a legal issue arise regarding the ownership of your work. The postmark will establish the date that the envelope was sent and, as long as you don't open it, it provides proof that the contents were developed by you at a specific time.
Keep records of any correspondence. If you are trying to sell your idea to a network or other outlet, remember to hang on to letters, emails or any written materials exchanged. But remember, if you're thinking of submitting your idea to TV producers, do not do so until you have protected it through copyright. It's best to first send a query letter to potential industry contacts asking them the proper channels in order for you to have your work reviewed by someone in their company that might be interested in producing it.