Spend some time at your local sporting clays course(s), and get to know both the regular shooters and the management. Tell them that you are in the market for a shotgun, and want to find the one that will best fit your needs. Typically they will invite you to "try my gun," and I urge you to take advantage of these offers, shooting both semi-automatics, over-unders, and pumps (since you expressed an interest in the latter). As a matter of courtesy you should offer to pay for any shells and targets expended. Do ask questions about what they like (and don't like) about their particular gun, and about any particular quirks or problems and how they were resolved.
Once you find a gun with which you shoot well, try it in a variety of target presentations and with more than one type of load (some shotguns pattern specific loads better than others, and the only way to find out is to shoot them). Finally, remember that the old adage "you get what you pay for" applies to shotguns, too. Pay particular attention to the fit of the stock. The late Gene Hill once said, "I would prefer, given the choice, to have my guns well stocked with fine wood rather than covered with engraving. I'd rather look good shooting than have what I'm shooting look good." To which I say, Amen!
Good luck, and good shooting! You've started down a wonderful path.