Hi there hartandsoncleani,
It's vital that all your transactions are correctly entered so that your books can be well balanced. QuickBooks Online is a great tool able to keep track of sales transactions so you can see the income your company made in a set period. I can point you in the right direction to see your invoices.
In order to see all invoices for a given period, you can go to the Reports tab, then scroll down to the Who owes you section. Click on the Invoice list, then you can put the date range to be for Last year, and you can even use the Group by menu to group them by month.
If you're looking to see other transactions for income and not just the invoices, you Can go to the Sales and customers section of the reports tab, and choose to run a Sales by Customer detail report, then change the Group by option to Month.
If you have any other questions, feel free to reach out here.