I just did January invoicing. How can I create a report just showing (1) the invoice totals this month and (2) compare with previous months?

QuickBooks Online has the report you need, dmlerner.

You can run the Sales by Customer Detail report. This lists the individual sales transactions for each customer, including dates, types, amounts, and totals. Afterwards, we can customize it to get the information we need. 

Here's how you can do it: 

1. From the left panel, click Reports. 
2. In the report search box, type Sales by Customer Detail. 
3. Filter the date to Last Year. Then, click the Customize button at the top right corner. 
4. Click Filter header to expand the list.
5. Place a check mark in the box beside the Transaction Type option and select Invoice from the drop-down.
6. Click Run Report.