I think Shopify and the custom mobile App that you want to create will be separate. So you will have your App developed yourself or with the help of a programmer. Then what you can do is promote the App through your Shopify online store to encourage people to use the App. Or, you can promote it within iTunes or other platforms though I’m not entirely sure how to do that.
To set up your products, click the “products” text link in the left-side menu. Then click “Add product.” This will take you to a page where you can enter your product title, images, variants (like sizes and colors), shipping weight and other details. You can also set a “Compare At” price, which will show next to the actual price on your site as a crossed out amount, letting customers know that they’re getting an item for less than the suggested retail price.