Assuming I have songs with lyrics already added to them, how do I easily view them when listening to a song?
With reference to the current version (iTunes 10):
Without adding third party software, you will need to actively select the currently playing song to view its lyrics.
If you open the Lyrics tab in the Info window, it will open in the same tab the next time you open it.
If you want to view the lyrics without interacting with your Mac, you might want to consider adding a third party visualizer. One such visualizer is Cover Version (freeware).
(NOTE: With OS 10.7.4 & iTunes 10.6.3, CoverVersion shows my lyrics with a upper case letter height of three pixels. I opened the info window and changed the font from 12 pt. to 144 pt., but it didn't change the Viz. But it's open source, so I could fix it if I were sufficiently ambitious.)
After you have installed the visualizer, you will need to select it in iTunes. For example:
View > Visualizer > Cover Version
Other Third Party Software
If you want to display lyrics while using your Mac for other tasks, you'll need to use a third party application.
There's no built in way without right-clicking/control-clicking the song and selecting Get Info but you can download widgets such as Desktop Lyrics
Get Lyrica from the Mac App Store, it's only $2 and worth much more. This will find lyrics for songs in iTunes and add them automatically to the song file. Very cool.
As to displaying lyrics automatically when the song plays... sounds like a feature Lyrica should add! I'll mail the developer.
I search a lot about this issue before and the best free solution I was found is to use Get Lyrical. it didn't use your lyric that added to them with iTunes.