These are the first steps you should take towards beginning your epic map editing journey.
Back-up your files before you edit anything!
To load up the editor, if using Windows you can find it under All Programs in the Start Menu, under TEdit.
You can also start the program from the folder it was installed in - it should be named TEdit.exe and display the TEdit custom icon:
To pan (move around) the view of your world use the scrollbars located at the bottom and side, or hold down the middle-mouse button and move your mouse around, or use the arrow keys Shortcuts and Keybindings.
To zoom in or out of the map roll you mouse wheel. You can also use Ctrl+= and Ctrl+- (these are the keys above and to the right of “P” on a Qwerty keyboard). The zooming will occur at the position of your cursor and won't occur at all if the cursor is not within the bounds of the world display.
Now that you have the editor up, you will probably want to check out the Basics, the Menubar, the Toolbar, and pages. At some point you will also want to check out the World Properties page and learn what can be done there to help shape your world. The Clipboard page will help you out if you want to learn how to use schematics.