What safety precautions to take before hiring someone to build the website

I’m new to WordPress and I decided to hire a freelancer to finish building my website. That includes designing and coding.
Besides the fact that I need to make research before picking the right and trustworthy person, I want to know(technically speaking) how to let him do the work without being hijacked or stolen.
My role is Administrator, what’s gonna be his?
I also know it has to do wit backup, FTP files, my host etc. but the whole process seems vague to me. Can anyone explain to me step by step how to execute the outsourcing safely?
I would really appreciate it.

Thanks