By pressing CTRL-SPACE you start selecting text. You can then kill text by pressing CTRL-w somewhere else, or copy text by pressing CTRL-Insert.

Often this text is not available outside emacs, for instance in a browser. To copy text from an emacs buffer to your Windows clipboard you need to press CTRL-SPACE twice instead of once. In emacs terminology you enable the Transient Mark Mode by pressing CTRL-SPACE twice.