The quick and honest answer is - you don't.
In games, as in many places in the software industry, ideas are worth nothing, only the implementation counts.
In addition, ideas tend to evolve during development and the design you end up implementing in the final game is quite often very different from the one you started with.
To be even more honest - it's likely your idea is not as original as you think. I'm not trying to suggest you're not creative, just that there are a lot of other creative people in the industry and we all have tons of ideas. I have written several game designs in the past, then found that other people implemented the same ideas a while later without knowing me. Sometimes I even discovered my ideas were implemented years before I had them by people I never heard of. That's the life in our business.
Another tip - don't try to sign people on NDAs just for hearing your idea. NDAs are fine if you expose them to code, art and data you have already prepared/collected. These are "implementation" products. Trying to sign people on NDAs for hearing your idea is a newbie move and no industry professional / investor will do it. It will make you look like an amateur.
Ok, so this was the discouraging part, now the encouraging one...
If you have a game idea that you think will rock - just go and try it. You don't need developers to try out a platform game idea, just use one of the MANY quick-builders tools out there. Game Maker is a good option as well as GameSalad Creator.
Often you'll find that what you imagined to be a cool idea might not be as fun to play. Sometimes you'll find that combining two genres into one game, as many ideas tend to be, misses the fun factor both genre had.
Games are about finding a fun factor. Identifying a pattern players enjoy recognizing and following over and over. This is immensely hard, but so rewarding when accomplished. I was in charge of the final project workshop of the Game Studies program in collage for 6 years. I went through this process with many students (including the fear of 'idea theft' at the beginning) - it is worth it every time a new game is born.
As to finding developers - look either between your friends or on internet forums (gamedev.net, indieteamup.com). Both places are good, but projects done with friends have higher success rate by far. You can also locate the nearest IGDA Chapter / Game Hackathon (like Global Game Jam) and befriend people there.
All in all, I hope you proceed with your game project. Feel free to contact me if you feel I could help in any way.
You really don't, the only real protection you have these days is the track record of the company you want to work with. If they have 20 lawsuits going on then I would probably stay away. The second protection is the first in on an idea and you really push it you become the front runners and get the lions share of the business. You can have a developer sign a non-compete non-disclosure agreement. Again reputation is everything, if a developer has a good one then that's the man you want to work with, call me if you have any further questions and I will see how I can help, thanks...Ken Queen