Love the 870, but good luck using one for skeet...the limited distance of clays in skeet don't make the lack of chokes a deal killer, but you'll have to be very fast with a pump action to hit doubles as well as O/U or autoloader shooters. As for trap, if your barrel does not come with chokes, even 16 yrd. trap will be iffy - not to say it won't still be fun, but just that your scores won't be what they could be.
I plan to get an 870 at some point, and am hoping to get a backbored, cylinder lengthened, ported barrel, and a Mesa Tactical adjustable buttstock and rail adapter. Probably a Vang Comp.
As for what to do before shooting it, all you need to do i clean the barrel and you're good to go.