Not sure what configuration you got, but if it was the version with only a pistol grip out back, the first mod is going to need to be a buttstock so you can use it for something.
As you said your dad got it for you, and assuming that as an older fellow he'd have known better than to get you a PGO version, you're pretty well set-up as is.
I have a side-saddle 5-shell holder on the left side of the receiver of mine. That's fine if you want a Home Defense gun with around 13 rds. on board at all times. An elastic butt-cuff holder that carries 5 or 6 would work instead, though, and be easily removable if you wanted to use it in the field. If you're going to hunt with it or shoot clay birds, please don't hang a lot of tacticool crap on it because you'll look like a ... well, a dork, at the range, duck blind, or deer stand.
If it is just for HD, then a light might be a real good idea.
-Sam
