You can use any gun you want. The only possible problem with a semi automatic is if it ejects the hulls directly to the side where they might hit the guy standing to your right - it's considered bad form.
Still, I only have one semi that does that - most kick them forwards at an angle, or drop them short.
As to over/unders. You don't *need* one, especially not when starting out. I tell folks who are starting just to get a Mossberg 500, Remington 870, or Winchester SXP - just a cheap field gun. Or look for an old 2.75" pump at a gun show. It's best to have removable chokes, though. I still enjoy shooting pumps. I shoot skeet, trap doubles and 5 stand with an old 870 when the mood strikes me - you are at no disadvantage with one.
Reason being is that a *good* over/under costs money. Do not buy a bargain level O/U. Buy a "B" gun - even a good used one, before entertaining one of those Turkish guns. Yeah the turks CAN make some great guns and I know a guy who shoots a Yildiz clone of a Perazzi, but those aren't "cheap" guns. You're looking at more than $2k for a decent used O/U of quality make, and probably closer to 3 in this market. It's not a purchase to make Just Because.
Anyway, that's my experoence. I just bought a new Beretta on sale, mostly due to the fact that the Browning 725 that I bought site unseen from a friend many years ago Just Because the price was right never truly fit me. I'll be selling that one in the spring