What is the best way to import automatically transactions that were made before I joined in?

Thanks for checking in with us, usercontact11.

If you'll want to import bank transactions into QuickBooks Online (QBO) before subscribing to it, we can manually download them in batches to your account. We can use the Web Connect file from your financial institution or import an Excel file. For the steps and details, check out the Import bank transactions in batches article. 

If you'll want to import your chart of accounts, customer or supplier contacts, journal entries, and bills, refer to these articles:

• Import customer or supplier contacts to QuickBooks Online
• Journal Entry Import in QuickBooks Online
• Import your bills in QuickBooks Online
• Import your chart of accounts to QuickBooks Online

I'd like to know how things going after trying the steps, as I want to ensure this is resolved for you. If you're referring to something else, please reply to this post and I'll get back to you. Take care always.