The 870 is the only shotgun I own, and it has killed MANY birds.....both the feathered variety and the clay variety. It's often referred to as a shotgun you can paddle the boat with on the way to your duck hole.
It's inexpensive, highly reliable, and nearly indestructable. For a beginner shooting clays, it would be a good start. You may want to consider an auto-loader or over/under for competition shoots, though. The pump action can be a distraction if you're aiming for quick target acquisition as in competition shoots.
But for a beginner gun, you can't go wrong. I believe a modified choke is standard issue with the 870, but it's got a threaded muzzle in case you decide to change. Don't bother with the 3 1/2" super magnum, though. I hear it's not nearly as reliable, and unless you're hunting Canadian geese, you shouldn't have any use for it anyway.