RE: First Shotgun
 

I think I'd stay away from the .410 as a first gun. Sure they're fun to shoot, but it takes a lot of practice to be proficient with one. There's nothing more frustrating to a new shooter than not being able to hit anything (not to mention greater possibility of wounding game, etc).
As far as the brand goes, I prefer Remington over Mossberg but either should do fine as your first gun. The 12 or 20 is really just personal preferance. I have both and use both but I prefer the 20ga. because it's a little smaller & a little lighter to carry in the field all day. There's really not a huge difference in the power or the number of pellets per shell between the 12 & 20. Obviously the 12 will reach out just a little farther and will throw a slightly larger pattern but the difference isn't all that great. I'd say if you're going to be hunting something like geese or ducks where you'll tend to have longer shots then go with the 12ga for that extra reach. But the 20 will be good enough for any small game, turkey, even deer (although at a slightly shorter range than the 12ga).

RE: First Shotgun
 

My 2 cents....I am just getting back into bird hunting and last year (after looking at alot of shotguns) ended up at Wal mart. I bought a Mossberg 500 combo for $250. It came with a slug barrel with rifled sights & a bird barrel with 3 different choke tubes. I used it alot this year for turkey, ducks, & pheasants. And starting tomorrow, I will be slinging lead at deer. I love my gun & would guess you would not be disappointed with it also. Also, the recoil is less than the standard 12 ga because the barrels are ported with holes. Good luck