Like with a PC, there's a Print Screen button that takes a screenshot of your...screen, do iMacs have something like that?
I thought command was the alt key... god damn... either way
key
Actually, it's "command"
Command + Shift + 3
(if we want to get all technical about it)
Don't forget: CMND + SHFT + 4
This gives you crosshairs so you can select what you want, instead of getting the whole screen. (For instance, just a website, or chat box, etc)
touché
BTW, it automatically puts the file on the desktop as a png file. If you need something else, you can always double click to open in preview and save as something else.