The first thing you’re going to want to do is to create your WordPress website within SiteGround. They actually have a really easy setup process to do that. I highly recommend following their tutorial that you can find here: https://www.siteground.com/tutorials/getting-started/start-new-website/
While I could do my own tutorial, this one is always going to be up to date with any layout changes, so I recommend referring to their website to do that!
Once your website is created and you’ve specified your new domain name in the setup process, it’s time to make sure your domain name is properly connected!