Brownings and Berettas feel much different but they are by far the best over/unders to start with. Try them and see which one feels best to you. DO NOT,UNDER ANY CIRCUMSTANCES buy a cheaper off brand gun with the intent to make it work for trap. It will be the most expensive cheap gun you ever buy. The sad voice of experience.
I would go along with those who have suggested an older model Browning Citori Trap with Invector chokes. Will serve your purposes and nothing can go wrong with them that Skeet's Gun Shop can't fix! I can't speak to the Berretta as I have never owned one. Martinpicker
Go to a large state shoot or the Grand, shoot everything you can afford buy the one that fits. As stated above don't buy the cheap off brands and take the one you want to a gunsmith (before you buy it) I would have someone like Phil of Phillips gunsmithing or Doug Hubbard. Then follow their advice! TAKE YOUR TIME! your dream gun is out there, find it. Good LUCK
A nice gun for a starter gun would be a Winchester Select Energy in 32" barrels. 30" barrels would also work. If you could find one with adjustable comb, it would be a bonus for shooting trap, clays and skeet.
If you are patient, you should find one for around $1,000 or less.
For a little under $3,000 new, you can get a gun like my current one, a Browning XS Special with high post rib. It has adjustable comb, and can be set up for all three games.
When I started 3 years ago I bought a 682 Gold e and love it, but like they all say make sure it fits, has adjustable comb, added 3/8 to the rib height and it shoots awesome,dont go cheap or you wont like it.
Cabelas has a used but mint looking 30" Special Sporting Clays edition (high post rib) for $1299 at one of their stores...IMO its one of the best all around clay target guns out there and older originals in as good shape as this one are getting scarcer by the day..and for once they aren't gouging you pricewise
If you are not looking to spend a lot, Beretta 686 Silver Pigeon I, Prevail I or Browning 725 (depending on which fits you better). If money is not an object, Perazzi MX8 (or 2008/8) custom built or Krieghoff K-80.