How to keep someone from stealing my business idea?

I have a great idea for an Iphone app. However, i know NOTHING about app development. I don't have time to learn about how to make apps, and i was wondering if it would be okay to search on Craigslist for someone who knows about development and could make it for me if i pay them. But then i wonder, what is keeping this computer wiz from just saying he doesn't know how to do it, and then turning around and taking my idea and using for himself. I know you can trademark stuff or whatever, but can you do that for just an idea? I thought you can really only protect something if it is already a certain thing.

Talk to patent attorney. You can only copyright the source code for an app.
Craigslist is a bad idea.
http://www.magnetmediafilms.com/blog/how-to-protect-your-brilliant-mobile-app-idea
(Until then, write down your idea and put it in an envelope and mail it to yourself and hang onto the unopened envelope).

Ideas alone don't mean much… Anyone can create something similiar, happens all the time when it comes to software.

The only real way to protect an "idea" is by keeping it secret. In business terms, this means using a non-disclosure and non-compete agreement that includes severe monetary terms if they blab or steal. There are lots of ways to do it. Patents become public knowledge when they are published 18 months after you file. Patents only cover inventions, not ideas. Trademark has nothing to do with this. Copyright protects only the "creative and original expression", not the ideas, concepts, functions, principles.