Make sure you don't forget this: After you are done with the Text Tool , put it away. The way you do that is to click the Pointer Tool when you are done.
Why go back to the Pointer Tool? Although you may not have noticed this, you use the Pointer Tool about 90% of the time in Visio. It lets you do one very vital task: Select. But if you keep the Text Tool active, you can't select anything. The only thing you can do is type text.
This is easy to forget. In fact, many people write to Microsoft in desperation, asking how to make Visio stop adding text. It's easy: click the Pointer Tool on the Standard toolbar, and all will be well.