If you have just downloaded the program, you'll need to unzip/install it (saving it to your computer from whatever site you got it does not make it immediately available to use). This should give you an icon on the desktop - when you run this, you'll be in the program/editor. When you get to that stage, check out the tutorials - there are some showing you around the editor, some showing you how to make different types of maps. Then once you've tried those, if you have more specific questions with things you're stuck on, you can get more help.