Go to your main menubar in Finder, select "Help."
When Mac Help window appears, Enter "screen shot" in the search bar.
You will be presented with a number of choices and methods for doing what you want (grab screen, a window, a menu, etc.).
Use the available Mac Help feature on your computer and also apple.com/support for answers to basic questions.
If neither of those resources serve up a useful answer, then come on over to the COW.