Hi laurban13,
I'm here to share more information on how to convert from QuickBooks Desktop 2012 to QuickBooks Online.
Before migrating, ensure to update QuickBooks Desktop to the latest release. Then, ensure the Total Target is less than 750,000. You can press Ctrl + 1 on your keyboard to verify this one.
From there, you're good to move your file. Since you have the older version of QuickBooks, you can move your data to QuickBooks Online with an online tool.
Once done, you're ready to use QuickBooks Online. I've added this article as your guide in migrating the data: Convert from QuickBooks Desktop to QuickBooks Online.
I've also added this link as an additional reference in getting started with QuickBooks Online: Getting Started With QuickBooks Online.
I'm just around the forum if you have concerns about the converting process.