The Marquee tool is selected by using the 'M' key. I would recommend doing one of the tutorials that is included with the program. In the Paint room, there are many options that don't seem to do much, until you know where to look, and describing that in detail here would be like me re-writing the maual. So, try out the tutorials, read the manual, and a lot of it will become more clear. Good luck, and have fun!