When shooting a shotgun for the first time, some folks have said, "It kicked like a mule". Well, I've actually been kicked by a mule once and the recoil of a shotgun has yet to come close to that experience. That's not to say that shotguns don't have some recoil... and it can be quite unpleasant for some folks.
There are a number of ways to reduce the effects of recoil from a shotgun including a good stance and grip, a padded shooting vest, holdin' it tight to the shoulder, and shooting a reduced recoil shotshell load like the Winchester WinLite Target Loads.
While some folks might recommend starting out with a 20 gauge... and I have to admit that a good 20 gauge auto-loader like the Benelli's our 4H Shooting Sports club has purchased make for a light, soft shootin' shotgun... but a 12 gauge with reduced recoil loads is very comparable in terms of felt recoil. We like to minimize the variations of ammunition we have to keep on hand to a minimum, and since almost all our shotguns are 12 gauge, it makes sense to keep some reduced recoil Winchester WinLite shotshells to take some kick off my gal's shoulders.
The recoil and report of the WinLites are significantly reduced over traditional loads, yet they still put enough shot downrange to bust clays pretty reliably. My daughter prefers shooting the 12 gauge shotguns with these loads over the 20 gauge shotguns. One consideration I should note... if you're running them through a semi-auto shotgun, make sure it cycles reliably with the reduced recoil loads. They didn't cycle reliably in a friend's Remington 1100, but did just fine in a Benelli and a Mossberg 930.
So if you're more interested in bustin' clays than bustin' shoulders... or if you're introducing a new shotgunner to shooting... you might want to try some... Winchester WinLite Target Loads.