Way to go! The first thing you should do is congratulate yourself on the accomplishment.
Once a buyer sends an offer that you'd like to accept, the next step is getting paid. You should start a transaction on Escrow.com for the agreed upon amount of the sale. Learn more about why you should only use Escrow to get paid.
The next step is to prepare your store to be handed over to your buyer. You'll want to make sure that anything you've agreed to sell is changed over to the buyer. This might include domain names, social media accounts, ad accounts, or physical inventory.
Once you're all set, it's time to request a store transfer. In Shopify, head to the Exchange app and click on the Transfers tab.