Purchase a Domain and Connect it to Shopify
Obviously, you are going to need a domain. Luckily, You should be able to get a .com domain from Shopify for about $14 per year. It's a very straightforward process and should only take You a couple of minutes to do. If your desired domain is already taken, then You can simply choose another domain suffix, such as .co, for example.
Third Party Domains
If you want to purchase a third party domain, You can head over to NameCheap, however, you should have the budget since some premium domains can go for up to $1,000.000.
To buy a domain from Shopify, navigate to Shopify Admin - Settings - Domains - Buy Domain
Connecting Your Domain to Shopify
For domains purchased in Shopify, there is no additinoal set-up needed. You are all set!
To connect a third party domain to Shopify, You are going to need to edit Your domain DNS settings.