Legal agreements (e.g. a Non-Disclosure-Agreement) are of course one possibility, however keep in mind that taking someone to court over a contract breach might be costly and take a lot of time.

Basically it comes down to trust, when you share your idea with someone, you should be able to trust them.

When picking a company to implement your project you might want to look for someone with a well established reputation and also someone that would not profit from stealing your idea. This means that your subcontractors field of business should be as far as possible from the area of your idea.