I'll echo what the others have said. it's difficult to line up a backing with the top. If you are determined, I would do as Peckish suggested, using a pin to go thru the layers to match up the top & backing. (Cut the backing border at least 3.5" cut off excess when all done quilting. ) Once you have enough pins in where you think it's matched, secure the sandwich with pins. A lot of pins. Normally you pin at a 4" distance. I would make that 2 or 3 inches. An alternative would be to hand baste it closely. You can use water soluable thread and quilt right over it. The thread will disolve as you wash it.
It's not a common thing to try and center the backing. If you hold the batting between your thumb and forefinger and rub, you can see that the batt has some movement. As you quilt, the layers can easily shift a bit.
For a first quilt I would definately go with a plain back.