"I’m afraid that my idea get stolen"You should find someone trustworthy and share it with them. I've had a few ideas which I think can be turned into huge companies - at first I was afraid to share, and after sharing, first with friends and family, and later with my university's startup office, I realized that no, they aren't interested in stealing your idea (the startup office actually advised the largest startup in the country, and they obviously didn't steal the idea, even though it wasn't too hard to duplicate on paper).
Obviously don't divulge it to people who can become competitors (like people here) before having executed at least a bit.
Nobody will be as enthusiastic as you. Heck, you'll have to explain your idea over and over, and shout it to the four corners of the earth once you want to start selling!
http://cdixon.org/2009/08/22/why-you-shouldnt-keep-your-star...
I don't know what the startup scene is in Germany, but if you're in a large city, there are hacker spaces, coworking spaces, meetups, etc. Not knowing anyone shouldn't be a problem, it would be ideal if you can enter a hackathon and establish some rapport and work together.
Speaking from experience, you have to be willing to work hard to make it succeed - my own project is languishing because I don't put in the hours (I work 10 hours a day). But put yourself out there, and validate early. Get it in front of customers.
I hired a freelance programmer and it didn't go all that well due to different expectations of his commitment. It would be great if you can build a crude prototype yourself.