Here is most of the source material for two of the Internet Safety talks that I've given on Thursday nights. I love Leo Notenboom's articles and don't mind admitting that I shamelessly stole from him.
The first part is on keeping your computer safe on the Internet. You Mac users don't have to worry about malware as much as we PC users, but viruses can still find their way to a Mac via a Word document or Excel spreadsheet with embedded macros (small chunks of code).
The second part is on how to read a URL and figure out where it's really taking you.