When you set up, you can turn on sample data, a few example customers and jobs so you can click around and see how everything fits before adding your own.
When you're ready for real work: go to Settings > Advanced > Remove Sample Data. It clears the example customers and jobs. Heads-up: if you set up with a trade, this also removes that trade's default job types, so you may need to re-add job types afterward. Your custom fields and anything you created yourself stay.