To add a part to the system so that you can later select it from the drop down list, go to General > Preferences > Jobs/Invoicing Module > Parts and Prices
You will be taken to the New Parts and Prices page.
Here you can enter a description of the part, a product code and serial number.
You will then need to add the cost of the item. This is the price you paid for the item excluding VAT.
You will then need to set a minimum markup percentage. This sets a lower cap for your sales price. (When you add a part to a job, you change the sale price, you will not be able to set it to be lower than the cost price plus the minimum markup)
You will then need to add a default sale price.
You will then see this part appear in the list below