Making sure your books are set up correctly in your QuickBooks Desktop is an important part of your bookkeeping. QuickBooks Desktop is a robust tool that can assist you with managing your business's finances. I can provide you some information on this.
In order to get your lists such as customers, suppliers, products and services imported into QuickBooks Desktop, you can follow the steps listed in this article showing you how to export them from QuickBooks Online and imported within QuickBooks Desktop. You can also use those steps to help you import your opening balances.
If you were to want to import more than just your lists, you can search for apps to help you with the conversion from within the My Apps section in QuickBooks Online. You can browse the selection of apps in that page and select the one that best suits your business's needs.
If you have any more questions, feel free to reach out here.