If you're of standard height and build, any make of pumpgun will likely fit you reasonably well. However, each brand is built slightly differently and you may find that one make fits you particularly well. Go to a gunshop and mount every pumpgun they've got. Then do it again a few days later, and so on, until you find one that feels right. Ask the salesman how you should be sighting along the barrel/receiver.
At this stage, don't be concerned at all about reliability, durability, accessories, etc. Any of the name-brand guns like Winchester, Remington, Mossberg, Savage, etc. that are in good condition will function well. Equally important will be removable choke tubes to tailor the pattern to the shooting, or a fixed Imp Cyl choke. Don't pick up a used Wingmaster with 30" Fixed choke barrel - you'll find it too frustrating to learn to wingshoot. You do need a 3" chamber; you DON'T need 3.5".
Finally, you've GOT to go practise. Either at a proper club, or in a field with some buddies and a hand thrower. Don't be afraid of the club; most guys are very helpful and will be glad to give you instructions as well as maybe even let you try their guns. You've got lots of time to learn how to hit things in the air before the season opens, but you've got to start NOW.
Good luck, and let us know how you're coming along.