Using keyboard shortcuts is the best way for beginners to become more efficient Mac users. Close? Open? Minimize? There is a shortcut for that.
Keynote is Apple's own presentation application. Like PowerPoint, it allows you to put on impressive presentations, but faster and with better end results.
If you keep your Mac in a public place like an office or a university, you should definitely add a password to your screensaver. It only takes a minute!
Guest logins is a convenient and secure way of letting friends (or strangers) use your Mac without letting them access your personal files.