if i give something to developer for making the app or website,how can i be sure that he will not steal my idea.

Answer 1

If you leave your car to the mechanic, how can you be sure that he wouldn't steal your car?

Well, maybe because it isn't worth becoming a thief, losing his job just for some random car. Similarly, a professional developer does not hold your idea to standards as high as being worth stealing.

Answer 2

There is always the chance that your idea get stolen. That's why programming it by yourself is the best choice.
But if there are parts you need help, try to checkout freelancer websites like fiverr and freelancer.com. These websites are trusted by a lot of people. Post your project or find a service or hire someone with the best review and quality to do your job. Most of the people with most and best number of reviews can be trusted.
But as I said, there is always the chance for your idea to get stolen.