hey Rocco-
these are pretty much two different sports, but you can get a gun you could use for both. Youve got tons of options. You can take the cheap way out, and buy something like a remington 1100 auto, and get a few different length barrels for it, for trap you'd pretty much want nothing less than 30 inch, maybe even 32, Sporting Clays, you could use the 30 but i'd say 28 inch would be a bit better. In trap you want a higher rib on the barrel than you'd have on the gun for sporting clays.
You COULD get the Browning Sporting Clays shotgun, its an over under kinda fancy beefed up version of the Citori. That gun would be decent in both events.
There are advantages to the O/U over the semi in those events. Especially if you plan on reloading. No picking up empty hulls if you've got the O/U.
Ideally, I'd say get two guns. My thoughts are you just can't beat the Browning BT99 in Trap.
Advice for someone starting out- do it once, and do it right.
Don't just go and buy the cheapest over under or semi you can find and think you'll get real good with them. I seen to many friends buy Stoegers and Mossbergs and be convinced they'll shoot as good as everyone else in Skeet / Trap / Sporting Clays, who are using Brownings, Remingtons, and K80s.
Try out a bunch of different ones, and don't be afraid of putting out over a grand for a decent O/U. They'll last you a long time.