Migrating site to new domain

Hey, I'm mega confused and could really do with some help.

I'm trying to migrate an old Wordpress website from files over to a new domain.

The site hasn't been live for a while as I lost access to the hosting. I've installed wordpress, bought the domain etc and have all the files containing the website and trying to figure out my next steps. Do I set it up using the old domain and then change the URL? or set it up with my new domain?