Hi there john-harvey-sail,
It's vital that all your information is correctly entered so that your books can be balanced. QuickBooks Online is a great tool able to import various information from files to save you time with data entry. I can point you in the right direction.
As mentioned by Fiat Lux - ASIA, you can import information using CSV and Excel files. The article linked explains which information can be imported, and which order should be used to make sure there aren't any errors. You can click on the specific information such as Bills, invoices, customers or suppliers in order to see the format needed for the file so that it can be imported, as well as other information further below in the article.
If you have any other questions, feel free to reach out here.