meguiar's comes in 2 flavors. mild and agressive.
with a paint job 13yrs old, a first time clay job won't do very much, the paint (if original) is oxidized, the paint has faded, it's battered and beaten up, there probably isn't any clearcoat worth trying on save on there.
However, just wash the car, then clay it, make sure you use the quick mist lubricant, some people like to use water, I like the meguiar's qwik mist. Buying the meguiar's clay kit is a better deal!!
just do the car in small sections, and break a piece of clay off and as you swipe along the lubricant you'll notice the area getting a bit smoother. the clay should be dirty, just kinda knead that into the half-dollar sized piece you tore. a little goes a long way.