How do I get dashboard to run on my Mac?

You're talking about a much-lauded program called Dashboard, and for the f12 key to bring it up, you need to have Dashboard running in the first place. I bet it is, actually, and that your f12 key is disabled. Let's check that first.

Go to Apple --> System Preferences... --> Dashboard and Exposé. You'll see this:

The last entry should be 'f12' as you can see in the figure above. If it isn't, switch it to be the correct value and close the window.

(Here's a tip too: Many Macs also use f12 as the CDROM eject key. If that's the case, you'll find that pressing the f12 quickly invokes Dashboard and pushing and holding it down for 2-3 seconds causes the CDROM to eject. One two, two functions. Who would have thought we'd end up with that interface error on a Mac!)

If indeed Dashboard simply isn't running, it might be that you have it disabled at the operating system level. Unfortunately, this is a bit tricky to fix, but it's doable. You need to start up the Terminal (Applications --> Utilities --> Terminal.app) and carefully type in:

Since it's critical you get it exactly correct, I suggest you just copy and paste what I have here.

If you DO want to disable Dashboard after you've tried it for a bit, you can use the opposite command in Terminal:

Either way, you'll want to log out and log in again to get the change to actually work, and then, if you have enabled it, you should be able to plug along and enjoy Dashboard to your heart's content.

There are lots and lots of Dashboard widgets you can learn about and download if you're into that sort of thing. A good place to start is the Apple site I reference above (Dashboard) but you might also check out Google's Dashboard Widget Archive, you can always go to reference Mac download sites like Versiontracker.com to see what's new, and finally, you can also put Dashboard widgets on your Mac desktop too.