Hi, brand new to online QB with no instructions. Mainly use it to write checks. Unclear if details must first be entered in 'pay bill', or do I just go to 'checks' tab?

I'm here to help you get started with your QuickBooks Online (QBO) account, @brown-commercial.

In QuickBooks Online (QBO), you can record a bill for your vendors that you'll be paying later. You can check this article for more information: Enter and pay bills. 

You can write a check or create an expense for expense transactions that you've paid immediately. If you need to enter a  check number, then you'll need to add your transactions by writing a check. If not, then you can add it through expense. 

Here's a guide on how to enter and write checks in QuickBooks Online and on how to enter, edit, or delete expenses in QuickBooks Online. 

Also, here's more information about what is the difference between bills, checks, and expenses. 

You can also check these video tutorials that can help you achieve most tasks in QuickBooks Online: Video tutorials for QuickBooks Online.

I'm always here if you need further guidance in recording your transactions, and or anything else. I'm always here to help. Have a wonderful day!