Can you give me advice on finding someone to make my website?

I have a website idea, and I don’t have a lot of money to spend. I need to find someone competent, obviously, and I want to be able to trust them with my idea. How do I guard against them simply creating a website with my idea for themselves?

The site will need to “do” things, so not just a page you click on, but people will sign up and create their own page.