My dad started me off with a 20ga pump and, I never even learned to shoot a shotgun until a few years ago and, I'm 40 now!
I've been getting on a soapbox and preaching about the pump vs. auto argument... So, why not here.
You say that you're interested in shooting the clay games... skeet, sporting clays etc.
At the range I go to, you don't see experienced shooters shooting pumps... ever. The only time you ever see pumps at the range is right before hunting season and during hunting season... after season is over, the pumps disappear. Then you see about as many autos as over/unders.
For a new shooter, in my opinion, an auto is easier to learn to shoot. They recoil less than over/unders and some of the best shooters out there are shooting them now.
For skeet, a 20ga and 12ga are about the same but, if you want to shoot sporting clays, you will have an advantage if you get a 12ga.
If I were you, I'd go and find a nice 20ga auto. It will be the easiest to learn with.