Check out the quote in Dadanox's sig. Nothing more needs to be said about shield. As Disc, you will most likely start most fights with PoM, then once the PoM bounces, shield, then every once in a while, between pulling Oreos apart and eating the middle, glance at the tank's health, throw out a Penance, and then a new shield.
Okay it's
a little more complicated than that, but the main thing to remember about a Disc priest is that your whole spec is designed around mitigating damage. Shield is your star player. It should be in every single scene.
I leveled a Disc priest with my husband's warrior (100% duo, never played alone) and it was just awesome in every way. The classes complement each other extremely well. It's like old school D&D.
Have fun.