I have a new iMac El Capitan (and I'm a new Mac user) attached to the Canon Pixma MX926 printer (about 4 years old) with a USB cable. I can print from the Apple help menu, but can't print any Word documents. I'm sure I'm doing something very simple wrong, but can't figure out what. Can anyone help?
Welcome to Stack.
Try reinstalling, or resetting up any drivers for your printer. You can do this using either built-in mac features, or using any drivers provided by Canon on their website (available here).
For the built-in Apple feature head over to System Preferences. Then navigate to Printers and Scanners. Click the - and remove your printer. Click the + and add follow the steps to re-add your printer.
Hopefully this fixes your issue!