Apple’s minimalist design means that the keyboard only has the keys you need most, everything else is performed with keyboard shortcuts. There is no dedicated printscreen key like you get on PCs, so you will need to resort to a handy keyboard shortcut.
To take a full resolution screen short of entire screen hold down Command and Shift keys, and press the number 3. To take a screenshot of a specific section of the screen, by cropping a box around it, press 4 instead.
This will make a PNG image file on your desktop, and will put the date and time into the file name.
Bonus: The PNG format is pretty good for screenshots, because it provides a good balance between file size and image sharpness, but if you prefer JPEGs instead you will need to input two commands into terminal (one by one).
defaults write com.apple.screencapture type jpg
The first line changes the screenshot format, and the second line restarts that process that is responsible for actually capturing the image. The default is png, but you can select gif, jpg, tiff, and even pdf.