Each time your business receives stock that you will be selling and that you'd like to track, you'll need to complete a Stock Delivery. Navigate to Stock > Input Stock Delivery > [Add New].
From your list of product lines, enter the amount you received of each. If you track gross profit, enter the unit price you purchased from your supplier for. You can use the filters section to help narrow down the particular product lines you received in your delivery. If you get certain product lines from particular suppliers be sure to update this in the product details, this can help you greatly when inputting a delivery. Just select the supplier, then only their product lines will show up on the delivery form.