First of all: Always create a Backup and try it on a Staging Shop first...Setting up a second Store is pretty straight forward. On the Multistore section click on Add new Shop. If you are using the same items as the current shop has add it to the same Group. Follow the onscreen instructions. Make sure you have added the domain to your Hosting.
Please be aware that reverting is extremely difficult after adding the store if you don't have a backup.