First off decide which sport you're going to do most. Sporting Clays, Skeet, and Trap all use a slightly different gun, mostly in the stock configuration and barrel rib. You can shoot all three sports with one gun, but you won't get the best out of your skills. There is even a difference between a Field gun and a good Sporting Clays. If you have to have just one gun until you decide which sport to concentrate on I'd buy either a good Sporting Clays gun or Trap Gun. I would not buy a Field Gun.
As mentioned above, Brownings are about as good a starter gun as you can find, many nice used ones out there and they come in all of the styles mentioned. Next would be Beretta's, also available in many makes and models. If you look you can find some nice used ones for decent prices.
Also don't overlook SKB's, Remington 3200's, and Winchester 101's. I'd stay as far away as possible from those cheap Russian, Turkey and low end Spanish O/U's. They may look nice, but quality is their big weakness.
I'm sure you'll get hooked on one of the shooting sports, and like everything else you'll start looking at what the "Good" shooters are using. If it's trap It won't take you long to long for a nice Krieghoff, Perazzi's ,Kolar , Caesar Guerini, or the real high end Silver Seitz.
There are a lot of places you can try before you buy, or shoot as many different types of guns as you can, a good start are your friends guns.
The one thing you need to remember, a well fitted gun is more important than an expensive gun, spend a few extra dollars to have one fitted to you and you'll shoot a whole lot better