If you don't already have web hosting, click here to follow along with the video to purchase hosting with SiteGround.
If you already own a domain name, here are the SiteGround specific instructions to either use the domain name with SiteGround or to transfer it over to SiteGround and manage it all in one place. Either option will work, you're just deciding if you want to manage the domain name and hosting from the same location or keep them separate.
If you're using hosting with a different company than SiteGround, they'll have instructions similar to SiteGround's on how to use an existing domain name with the servers or how to transfer the domain name. Just do a search for the above links replacing "SiteGround" with your hosting company's name to find those resources.