Q: how to take a screen shot

Command ⌘ + Shift 3 or 4

3 for full screen, 4 for cross air selection

More here, including windows etc.

10,001 Mac questions answered

Import into Preview and save as a different format like jpg.

Also if you want to pernamently change the screen shot format, you can use Tinkertool or OnyX from Macupdate.com

(10.8 users hold control key or right click to "open" non-Apple tied software from the web and elsewhere)

Shift+Command+3 for full screen, Shift+Command+4 for cursor to choose area.