An easy way to make text look like it is being written on the screen is to use the Stroke effect.
First, create your text layer. Then, make a mask path by creating points that flow in the manner you would write the text. For example, finish the word before going back to dot “I’s” and cross “T’s.” Be sure you mask path does not cross over any parts of letters out of sequence.
Then, add the Stroke effect to your later. This is found under the Generate family of effects. By default, the effect will use your mask 1 as the path. If you have multiple masks, you can select the “All Masks” checkbox. On the Paint Style option, select Reveal Original Image. This will only reveal what your mask path is outlining.
Increase the Brush Size to thicken your outlines. You’ll begin to see your text form. Set keyframes for the End value to animate the write-on effect. Fine tune the mask by adjusting your mask points as you check your animation frame by frame. You can also use your End keyframes to perform actions like pausing the animation between words.