You can access the Product/Service builder from under the customize menu.

Start by clicking the 'New Product/Service' button (system language may vary and you will only see the create button if you have the permission to create products/services).

On the create Product form, you just need to add some basic required info like name, product type (service, rental, or retail), price, and the related product category assignment. Click 'Create' once you have the details filled-in.

If you create a Retail or Rental product, you will see fields where you can set your quantities of stock as well as retail and rental prices.