There are two ways to receive stock. Direct Stock Receive and Purchase Orders (PO). Before receiving stock make sure you have already created your products.
Direct Stock Receive
Direct stock receive does not require a PO, and you can directly receive the stock from a supplier.
Goto Inventory > Stocks > Direct Stock Receive
Choose a location you want to receive stock to. You can search for products and enter quantity to receive.
You can receive stock from a Purchase Order also. In this case you should first create a Purchase Order, and send it to the supplier. Once you have received goods for the Purchase Order you can receive goods by creating a Purchase Receive.
First goto Inventory > Purchases and click "+ New" to create a new Purchase Order.
Once you've created a Purchase Order add products to the PO.
After you've added products to the PO, you can Approve the Purchase Order to finalize it by clicking "Approve" button the top.
After a PO is approved, you can receive goods by clicking the "Receive Items" button in the PO details page. Once you enter the quantities and receive items the inventory will be added to the system for the product.