The easiest way to do this is to keyframe the text media like this:

Step 1: Insert a Text Generated Media and set up the font, size, color, etc that you want.
Step 2: Make sure you press the Left Justify button because text is center justified by default. If the text is centered it will move as you add more letters.
Step 3: Start by typing the first letter of the text only.
Step 4: Now make a keyframe a little further down the timeline and type the 2nd letter.
Step 5: Make another keyframe further down the timeline and type the 3rd letter.
Step 6: Keep making new keyframes and typing one letter at a time until your whole message is written.
Step 7: Finally, highlight all the keyframes and right click on one and select Hold from the popup menu for the keyframe type. This will cause the letters to instantly appear at the designated keyframe.

That’s all there is to it. Add a typing sound effect and you're all set.

~jr

For a different effect...

1. Use default text, type a whole sentence at once.
2. Stretch out the event a little.
3. Drop in the Statish 3D Plug-inPac filter
4. Set first keyframe 'crop right' so it covers all but the first letter of your sentence.
5. Set last keyframe 'crop right' to zeo.

Adjust length to change speed.