To print invoice select one from invoices list and clik button. In new window you can choose print template for your invoices. Templates are little vary in presenting data and before print you can display preview and select appropriate for you.
You can also save invoice as PDF document.
Invoices are printing in A4 format. If your printer does't want print after click button, select preview and from upper toolbar click Print Button. That displays System printing window which you can use to print, it should help.