I am an entrepreneur in San Francisco and am afraid to tell people about my business idea without ensuring that I'm legally protected first.
Obviously, if you can trademark or copyright your idea, you should do so. They are easy to do, and the government website gives you all the information to do it.
But most wanna be entrepreneurs just have an idea. The first thing I tell people is to google the hell out of your idea. Search it every which way possible. If it's a product, search for it on Amazon too. The fact is that most ideas are not really so original that no one has thought it up before.
More likely, there is already a company out there doing something similar. Generally speaking, if your idea is in the area of social media, shopping, restaurants and bars, education, or travel, it has been done already. Parking apps are also popular today, and nearly every city in America has a startup along those lines.
If you have found a company doing it already, then dig a little deeper to see if they are achieving any revenue or some success. if not, then investigate why not. This will only help you -- even if you continue with your idea, you can and should learn from the mistakes of others.
Look at their management team: Do they have people with deep domain experience or contacts? If you don't, then they will likely beat you out.
If there is no other company doing anything remotely like yours, fear not. No one is interested in copying your idea because they are too busy pursuing their own dream.
Now, that isn't to say that your idea won't be copied, but it really isn't the big deal people make it out to be.
The real worry, though is that if your idea is that easily copied, then my question as an investor is what will stop some bigger company, such as Amazon, Google, Apple, Groupon, or who ever, from starting their own company and putting you out of business?
I have seen exactly that happen to many companies. Every one of them swore that no one could possibly copy their idea because ......And then a few months or a year or two later, guess what? That is exactly what happened.
Bottom line: If you are that scared of having someone steal your idea, it's not a very good idea to begin with and one you will find few investors.