It might sound superfluous, but the first and perhaps most important question is: What do you want to show? A screenshot should limit itself to exactly that item which is important. If you want to show a task requiring several steps, consider taking several shots and arrange them as a sequence.
In our tutorial, we want to create a screenshot showing the "Settings..." menu item. You can see the final result of what we produce below or in the section about project settings of the HelpSetMaker manual.

The settings menu item screenshot