You can create a new product or service line item while you working within an estimate or job. In the products and services section of the page, click the 'Create 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.

Once you create your new product/service, it will then be selectable in the 'Add Line Item' button section if you want to add it to the current estimate/job. The item is now part of your master product list.