The easiest way is to create a New FTP user for the domain where it’s hosted. Then give them the FTP password so they can upload themes and plugins. And, of course, the WP Admin login.
This gives them the keys to that kingdom. If you don’t trust that person, you should find someone else.
Better yet, do it yourself. You’ll learn tons and be pretty self sufficient. Whenever you get stuck, you can post here or in the WordPress forums. There’s a huge support community for WordPress eager to help out new users.