To take a screenshot ingame or in the World Editor, press the "print screen" button (it should be next to backspace.) Then open your screenshot folder, which is located in your Warcraft 3 folder, and search for the correct file among your screenshots. If you cannot open the files, download Irfanview from the tools section.
To find the correct camera angle, simply hold your Ctrl key down, and drag your right mouse button around your screen. If the sky is zoomed to close, hold Ctrl and roll in/roll out. To zoom in or out, use Shift and right click.