I'm Miles from the Social Care team at Shopify. This is a great question. I'll be happy to help fill in the blanks for you.
When you sign up to Shopify, you're given a .myshopify.com domain as part of your subscription with Shopify. This domain acts as an account number as well, and it's what we use to identify an account. With that being the case, when you're logged into your admin, you'll always see that .myshopify.com domain. It is important to note that your customer will never see this domain, as long as your new custom domain is the primary domain. We've got a great guide centered around managing domains that I would suggest taking a look at to help understand it a little more. This will walk you through what a custom domain does as well as how to make sure it's set up correctly.
If you're new to Shopify, a great place to start is our introduction guide, which will walk through everything from signing up to making your first sale. You can check it out here. This guide will walk you through our pricing plans, an overview of Shopify, getting set up to start selling and hiring help fi you need it.
I also recommend checking out Shopify Compass. Shopify Compass provides you with free webinars and courses on things like store design, marketing and more.
I'd love to hear more about your business as well. What pushed you into selling your products online?