When you have products that you want to sell but are not yet in stock, you can use the Pre-Order functionality that is build right into the theme.
The first step is to create a "pre-order" collection. From your Shopify Admin > Products > Collections, you can create a new collection called Pre-Order.
All products in this collection will show with a Pre-Order Banner on the the collection page.
The Add-To-Cart button on the product pages will also be replaced with a Pre-Order Button.