What you will need to do is, first create collections for each of your brands.
If you need help creating collections, here is the article on Shopify Help: https://help.shopify.com/en/manual/products/collections
Once you've created the collections for each brand, you can add a menu item labeled "Shop by Brand" to your main menu, with each Brand collection linked under it.
If you need help adding/editing your store menu here is that article: https://help.shopify.com/en/manual/online-store/os/menus-and-links/editing-menus
You can also add a collection list section on your home page (with most themes) with the heading "Shop by Brand" and the brand collections linked therein.