Did you register it here? If so just go to Websites > Site Domains and click on Move for your old site. Select to A Domain I Own and choose it from the pull-down menu. Click I'm ready, move my site and you're done.

If it's registered elsewhere you'll need to either point it to WA or transfer the domain to WA. Let me know if you need directions for doing that.

Hey Sheryl, welcome to WA. Please read these links may help you as well: My domain that I own is offline how to add it to wa? How to Add WA Hosting to WA Registered Domain Telma

You create a new website (Websites then click the + button).

The select the On a Domain I Own button.