Don't show it to anyone, unless you have applied for your patent or you will never get a patent on it, if you have to show it to anyone, then here is what you must do.
Do a full drawing and explanation of your invention how it works, anything you can think of to show its your idea, add the date, go to the post office, stick it into a registered envelope and post it to your self, when it arrives on your door step DO NOT open it, and store it somewere safe, this way you have full legal proof of your invention and the date if anyone try's to nick your idea.