To pay a bill, you have a few options starting at the home screen. The most direct way is to click on an unpaid invoice on the Recent Invoices panel.
You can also go to the full invoices list by clicking view all on the bottom of the panel, or click billing in the sidebar, then clicking My Invoices.
From the My Invoices page, you can search, sort, and filter through a list of your invoices. When you have found the invoice you wish to pay, click it in the list to go to the Pay Invoice page for that invoice.