Do you want to take a picture of your screen? How about a video recording?
These instructions are maintained over here. This is a wonderful tool everyone should have on their computer. Need help with a problem online? Take a quick video showing what's going on and you'll find it's so much easier to get answers when you can show the problem.
There's probably a PRT SC button on your keyboard. You can press that and it'll copy the contents of your screen to the system's clipboard. That means you can just paste the picture in an email if you want to, for example, show someone a problem you're seeing. You can also use the built-in snipping tool.
You can press CMD+Shift+3 to create a screenshot of your whole screen on your desktop. Or you can press CMD+Shift+4 to select a particular area of your screen.