After the startup wizard has run, you will need to create at least one Salesperson. Ideally, you should also create Item entries for each of the items you will be selling, so that they can be easily added to a transaction via the item code. All default settings should work "out of the box". See the Common Tasks topic of this manual for help on creating Salespeople and Items.
Once you have created at least one Salesperson, and at least one item, you can experiment with creating transactions using the main screen. Don't worry about creating false data - you can easily remove the test transactions using the Delete action in the Transactions List.
When you feel confident creating transactions, you can fine tune Copper using the configuration Options (from the Tools menu). This lets you specify more details about your business, tax rates or the layout of receipts.