If you've already determined that the "Dissolve - dither" animation preset does what you want, why don't you just use that? If you want to build such an animation yourself, you can apply the animation preset and then look at how it's put together. See this page for information about animation presets, including suggestions for reverse-engineering them.
BTW, if you're new to After Effects, I very strongly recommend that you begin here:
I applied it to the text layer I was using.. and I looked at how it was made.. the problem is that it is a transition preset, and I don't want it to affect the background, just the text i'm using.
Set the blend mode of the text layer to Alpha Add then animate the opacity. That will give you a nice looking dissolve.