Apple has a page listing dozens of standard keyboard shortcuts you can use at start-up, with the Finder, and with many applications. If you’re a keyboard person, rather than a mouse person, you may find some shortcuts here that you didn’t know. At a minimum, it’s worth bookmarking this page to have a list of keys to press at startup, if you need to change boot disks, boot in Safe mode or boot from an optical disc (if your Mac still supports that).
Dan Rodney has also composed a friendlier list here