As Mylenium so concisely indicates, this is a question that shows that you are quite new to After Effects. Everyone is new at some time, so there's no problem there. It's just that it's usually best if you do your homework and use the getting-started resources provided for you before you ask a lot of specific questions.
To answer your question, here's a relevant sentence from "About animation, keyframes, and expressions":
"For example, you can animate the Opacity property of a layer from 0% at time zero to 100% at time 1 second to make the layer fade in."