Last night I sort of impulsively bought my first gun, I went to Dick's and got a 12ga Mossberg Maverick 88 with the 18.5in barrel. I'm thinking I want to add a flashlight, pistol grip/folding stock and some sort of sight. Is there anything that anybody would recommend?
Yeah, skipping the folding stock, pistol grip, flashlight, and use the sights it has.
Go learn how to shoot it first. The rest of that stuff is a waste.
Build it as your skills grow. pistol grips on shotguns aren't all that great, but required for folding stocks, which also aren't all the great. For HD use a light is a good thing, depending on how its used.
Red Dot sights (Aimpoint/EOtech/many others) are awesome, but you still need to use iron sights effectively first. Batteries die, optics fail or break, plan B is iron sights, and if you're in a situation where you take aim and need to remove your main optic and use irons you're already behind the 8 ball.
Build up your skill set, then buy gear. All the mall ninja gear in the world is useless if you can't run the gun under stress. But a skilled shooter with even a stock gun is a force to be reckoned with.
Leave the regular stock on, pistol grips suck for shooting, especially 00 buck or high base magnum turkey loads. If its just for fun and Home defense pull the plug out of it, just dont take it into the field for hunting which i doubt you would do since it has no provisions for a choke if I remember right.
As has already been mentioned, don't bother with all those bells and whistles. Most of it is useless. Practice shooting the shotgun and learning how to manage the recoil. Learn the characteristics of the different types of common ammo for it (Bird, Buck, Slugs). Get proficient at combat loading it.