how to protect my idea from stealing?

Hello every one:

I have an idea for a new 2 websites that work together.

The first website will provide a free service to his members while i use the second site to get profit from the first site.

The problem now is i will hire some developer to make those sites, but how can i be sure that the developer will not steal my idea and make his own sites using my idea.

How to protect my idea from been stolen by that developer?

Thanks.