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.
I've discussed ideas with hundreds of startups, I've been involved in about a dozen startups, my business is at $1M+ revenue.
The bad news is, there is no good way to protect ideas. The good news is, in the vast majority of cases you don't really need to.
If you're talking to people about your idea, you could ask them to sign an NDA ("Non Disclosure Agreement"), but NDAs are notoriously hard to enforce, and a lot of experienced startup people wouldn't sign them. For example, if you asked me to sign an NDA before we discussed your Idea, I'd tell you "thanks, but no thanks".
This is probably the right place though to give the FriendDA an honorable mention: http://friendda.org/.
Generally, I'd like to encourage you to share your Ideas freely. Even though telling people an idea is not completely without risk, generally the rewards from open discussions greatly outweigh the risks.
Most startups fail because they build something nobody wants. Talking to people early, especially people who are the intended users/customers for your idea can be a great way to protect yourself from that risk, which is considerably higher than the risk of someone taking off with your idea.
Another general note, is that while ideas matter, I would generally advise you to get into startup for which you can generate a lot of value beyond the idea. One indicator for a good match between a founder and a startup is the answer to the question: "why is that founder uniquely positioned to execute the idea well".
The best way to protect yourself from competition is to build a product that other people would have a hard time building, even if they had 'the idea'. These are usually startups which contain lots of hard challenges on the way from the idea to the business, and if you can convincingly explain why you can probably solve those challenges while others would have a hard time, you're on the right path.
If you have any further questions, I'd be happy to set up a call.