I would say get an agent, and have the agent deal with one of the production companies that already serves Discovery Networks or whatever.
Anybody can steal your idea for free and get away with it, the “secret” is to lock up the specific assets needed to execute the specific expression of your idea. You would need to ink exclusive contracts with all the key people and content experts having to do with your idea, that they will only do TV thru you.
And this means of course you’re going to have to advance these people some money, just to sit on their hands and not talk to any other producers. Whether your show gets picked up or not.
Even then, nothing is stopping a rival producer from developing his or her own, independent resources to execute the same show idea. So, the more generic the concept ( home improvement, auction finds, pawn shops, funky jobs, cooking, singing/amateur performance contests, gardening, man versus nature endurance tests, etc.) you have, the easier it is for someone else to mimic it with their own exclusive talent. And there’s nothing you can do to stop it, except to make and market a superior, iconic version, and hope the public favors you, and spurns them.
If the show idea is so unique that there is only one person in the whole world that can host it or whatever, NOW you have something you can lock down with a contract and pitch to a network, and they might try to imitate it, but they can’t duplicate it. And you can sue them if they try then, because you’ve got contracts in place.