Always maintain both sets of books before deciding QBO is the right version for your business.
Then, start the conversion process by logging in as the admin to both programs.
Open the file by going to the Company menu.
Choose Export Company File to QuickBooks Online.
Follow the on-screen instructions to connect to QBO.
Once done, connect to QBO and import your data. I'm including an article which will guide you through this process from start to finish: Convert QuickBooks Desktop file to QuickBooks Online. For example, it contains links on installing a trial account of Desktop should you not have a version available and how to restore the file in order to import.