Probably a simple question, how to set up a “test” site

Hi, I’m pretty sure I know how to install WordPress on a server from watching YouTube videos, although I haven’t tried it yet. But what I’m trying to do is take an existing website and convert it over to a WordPress site so the owner can add content himself. My question is, how would I install WordPress so I can work up the WordPress site while the existing site is still at the url, then switch it once I get the WordPress site finished so that the url now displays the wordpress site? (for example, if the site is at yadayada.com, I’d like the present site to be displayed there while I work on the WordPress site, but once I finish the WP site I’d like yadayada.com to go to the new WP site).

I hope I’m making myself clear, any advice is greatly appreciated, thanks!