How do I add lyrics to an animation?

What do you mean by animating the lyrics? Are you thinking something karaoke style where there are words at the bottom of the screen and they highlight as the song is being sung? If so, you can make that effect with a mask.

Import your song and insert keyframes on that layer for the duration of the song. In the Properties panel, set the audio Sync to Stream, so you can scrub the Timeline to hear where the words are. Add a layer and enter the text for your line of lyrics on the stage. Make this text black (or whatever color you want to initially be seen). Insert keyframes for that layer so that the words are on screen for that portion of the song. Duplicate this layer (which puts a copy above your current layer).  Change the color of the text in the lower layer (original) to whatever you want the highlight color to be. Add another layer above both of the text layers and place a rectangle over your words. If you want it to reveal the line in one motion, convert this rectangle to a symbol (it can be either a graphic or a movie clip) and create a Motion Tween. On the keyframe where the reveal should finish, insert a keyframe and move the rectangle so that all the words are exposed. Now make that rectangle layer a mask (right-click on the layer and select Mask.

If you want the animation to expose one word at a time, instead of a Motion Tween, you could insert keyframes at the beginning of each word and move the rectangle to expose it. Would wouldn't actually need any tweens in that case. Then you could make make the rectangle layer a mask.