My Illustrator document has a transparent background and some green guidelines. How can I get rid of these and return to a normal document?
That is how a document created with a "Video" profile is set up.
Turn the transparency grid on or off through the View menu (View → Show/Hide Transparency Grid or SHIFT+cmd+D).
The green guides are the video safe areas and center marks, you can turn them off by enabling the Artboard Tool (SHIFT+O) and toggling the switches in the control bar: