If you are planning to vet your idea with legitimate entities, they should not have a problem with executing a non-disclosure agreement with you.

From the source...

"A non-disclosure agreement (NDA), also known as a confidentiality agreement, confidential disclosure agreement (CDA), proprietary information agreement (PIA), or secrecy agreement, is a legal contract between at least two parties that outlines confidential materials or knowledge the parties wish to share with one another for certain purposes, but wish to restrict access to. It is a contract through which the parties agree not to disclose information covered by the agreement. An NDA creates a confidential relationship between the parties to protect any type of confidential and proprietary information or a trade secret. As such, an NDA protects non-public business information.

NDAs are commonly signed when two companies or individuals are considering doing business and need to understand the processes used in each others business for the purpose of evaluating the potential business relationship. NDAs can be "mutual", meaning both parties are restricted in their use of the materials provided, or they can restrict the use of material by a single party."
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A common tendancy among entrepreneurs is to believe that your idea alone has some sort of intrinsic value. As a result, many people with "ideas" don't want to talk to others about it in fear that someone might steal their idea. The reality is that ideas are a dime a dozen. Lots of people have lots of ideas. What really seperates the talkers from the doers is actually trying to execute on that idea. Another common tendency is to over complicated an "idea" too early by trying to add to many bells and whistles. I am very much in favor of a more open approach. I'd encourage you to talk to people about your idea. The reality is that you'll need help executing on it down the road, so why not get some free feedback from your friends/other people that might actually use the product down the road. Most people aren't smart enough to know what the market wants, but the good entrepreneurs are smart enough to listen what the market is telling them and act accordingly. That means get a simple version of your product/idea up and running and listen to what your users tell you. Make improvements based on those recommendations.

Back to your original question, your "idea" alone is worth nothing. That might be easy for me to say and hard for you to hear, but it is true. The sooner you realize that the better. I think you'll be surprise about the feedback you'll get from others if you talk to people about what you are thinking about. They might be able to provide a perspective/suggestions that you hadn't considered previously. I'd say that a much larger percentage of people want to see other people be successful rather than look for ways to steal from other people. You'll get a lot further including other people in your thinking that you will holding your "idea" close to the vest and never sharing it. Open is where its at! Good luck.

Put your entire idea down on paper, place it in a sealed envelope, and mail it to yourself.

The postmarked, unopened envelope is proof of the conception date and source of your idea.

File a patent or provisional patent on it. I have a few patents pending in social media. It's getting tougher with the patent office, but it's worth a shot.

Then have everyone sign NDAs or non-disclosure agreements who you talk to.

If your idea is in the area of social media, the chances are remote that there is anything patentable or even hard-to-replicate about the basic idea, or that no-one else has thought of it.

Is Twitter the only micro-blogging site, or Youtube the only video-sharing site?

Executing brilliantly will most likely be far more important than having a good idea. So don't go overboard with secrecy, talk to people that you trust to see what they think of the idea.

If you get around to talking to people like VCs, they will not sign non-disclosure agreements or anything of that sort. They see too many ideas from too many people with too much overlap to want to leave themselves open to lawsuits of that kind.

The short answer is that you cannot prevent it from being stolen. You can use an NDA as suggested, but that won't actually prevent someone from taking your idea and using it (though it might discourage them).

More importantly, take action on your idea ASAP. If you don't, someone else will independently come up with the idea and act on it. If your idea involves a new invention, method, or system, you may be able to patent that implementation of your idea, but an idea itself is not patentable.