My MB 2008 crash randomly, how to use activity monitor?

My MB 2008 crash randomly, how to know why?

Hello, my macbook unibody late 2008 turns off randomly, most of the times I'm not even using it, wether it's on battery or connected. I'd like to know how to use activity monitor to know why the **** this stupid thing happens.

It happened twice this morning, I turn the macbook on and then I go to the activity monitor, then what? How do I know what to look at? I'm sorry I'm not very smart with computers.

I'm on Lion.

When I go to

~/Library/Logs/DiagnosticReports/

I can't find any report from today's crash, just the last one from december 22 and an older one from september. Why is that?

Try Applications/Utilities/Activity Monitor.

Also consider Console from the Utilities folder.