To point your domain to Siteground, you have two options: by using Siteground nameservers or using A record.
Option 1 - Siteground Nameservers
In order to gather the required nameservers, you will need to log in to Siteground. Once there go to your Client Area > Websites > Site Tools.
You will be able to see your website nameservers on the Site Information section.
If your domain is pointing elsewhere by NS records, your DNS Zone management is moved to the provider you pointed the domain to and should be managed from there.
Option 2 - Siteground A record
Alternatively, you can point your domain to Siteground via A record (it can be copied from the same Site Information section under website IP). Once you have it, all you need to do is add the A record here at your domain’s DNS Zone.
After you specify a new A record, propagation will start which can take up to 24 hours to fully propagate.
That’s it! Now you know two ways of pointing your domain to Siteground.