You can' t say it any better than akbound did.

If you' ve never shot a 12 gauge before, start out with target rounds so it won' t knock you around so bad. It' s not a terrible kick, but if you' ve never shot it before, it' s better to work into it slowly.

A 12 will kill anything under the sun - squirrel loads all the way up to 00 buck or slugs. The only thing you' ll have to move with is if you start shooting outside 75 yards on big game - then you' ll start looking at rifles. Sounds like you' ve got a great plan for a full-grown beginner, and you' ll love your 870, trust me.