The best accessory for that shotgun is a well trained and well practiced owner. In other words, trigger time for YOU. That's a perfectly useful shotgun you have there right out of the box, once you give it its first cleaning as the owners manual recommends that is.
Louis Awerbuck's short list (from memory, don't see my class notebook right now) of accessories for defensive shotguns is:
- a stock short enough for the shooter to manage comfortably
- a sling
- a white light source
- sights, if you need them
And the above applied intelligently, with the knowledge that reliable function of the gun trumps any accessory.
I won't argue with any of those and even maybe a couple more (I like Sidesaddles, some don't, and I like the XS Big Dot bead epoxy-over for factory pedestal beads -
http://www.xssights.com/store/shotgun.html , again some don't like them), but you can take or leave the tons of tactikewl stuff that's available out there for 870s as you will. It's your gun and your money.
But the best advice I can give you is to not try and substitute hardware for software. In other words, you cannot buy skill in a blister pack and bolt it onto your gun. Skill takes training, time, sweat and practice. And there is no substitute.
Stay Safe and have fun,
lpl