If you have products that you would like the customer to see that are related to each other go to Products in your storeadmin. Edit the item you want to add related products to. Scroll down the page and click add options, on the next page scroll down and click Add Inventory, on the next page you will see Related Items. Here you can choose up to 5 products that are related to your item from the drop downs. Click save and preview the product on your site to see that links have been added to allow a customer to click and purchase those items to if they wish.