Q: how can I make a snapshot of my screen?

• Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
• Command-Shift-4, then select an area: Take a screenshot of an area and save it as a file on the desktop
• Command-Shift-4, then space, then click a window: Take a screenshot of a window and save it as a file on the desktop
• Command-Control-Shift-3: Take a screenshot of the screen, and save it to the clipboard
• Command-Control-Shift-4, then select an area: Take a screenshot of an area and save it to the clipboard
• Command-Control-Shift-4, then space, then click a window: Take a screenshot of a window and save it to the clipboard

In Leopard and later, the following keys can be held down while selecting an area (via Command-Shift-4 or Command-Control-Shift-4):

• Space, to lock the size of the selected region and instead move it when the mouse moves
• Shift, to resize only one edge of the selected region
• Option, to resize the selected region with its center as the anchor point

http://guides.macrumors.com/Taking_Screenshots_in_Mac_OS_X

Press Shift-Command-3. The snapshot will be on the desktop.

You're welcome.

If you don't want to have to remember all those shortcuts, you can do most of them using regular menu commands in your Preview app or in a utility called Grab that also comes with your Mac.

Grab is installed with other Apple utilities, so it is in your Utilities folder.

Tip - You can find & instant launch any program on your Mac by typing its name into Spotlight search, so it doesn't have to matter if you don't know where it is.