If you are using Standard or Average Costs, you can adjust the product cost using Cost Adjustment window.
To check your current costing method, see Accounting Schema window.
The default for a new installation is Avarage Invoice, and this cost records are created when completing a purchase (purchase order, invoice vendor and material receipt). After that, you can adjust using Cost Adjustment window.
Or you can change costing method to Standard and do a direct adjustment to your products.
By the way, you may be interested in these general guidelines to setup an initial client: