First off, what is a screenshot? A screenshot is a picture that shows whats on your computer screen.
1. First, get what you want on your screen.
2. Locate the PrtSc/Sys Req button on your keyboard.

^ for most desktop computers, this is where it should be.

^ and that's where it should be for most laptops. :]
3. Now then, get what you want on your screen and press the PrtSc/Sys Req button.
4. After that, open up paint.
5. Click on the Free-Form Select tool.
6. Right click on the paint area, and click paste.
7 Save the picture and voila. There's your screenshot. You can crop your image (means only select areas you want and get rid of stuff you don't) by clicking on the select tool, draw a rectangle around what you want, right click and copy it, open a new paint, paste the new image, and drag the boundaries so it fits the image.