Buy a gun that fits your hand, first and foremost. Something that doesn't fit will never be comfortable and its unlikely that you will ever become proficient with it. Go to a store with a large selection of guns and try them all, the one that fits will stand out readily. Then see if its available in a common caliber that you think is applicable to your needs, is reasonably inexpensive to shoot and preferably something you can reload for in the future.
I tend to stick with common military calibers (9mm, .45acp and .40sw that is now common with most lawenforcement) as surplus and commercial ammo is usually readily available at a better price than some odd calibers like .357Sig or 10mm.
Most of all, find something that is not physically punishing to shoot all day and practice, practice, practice. Shooting one box of fifty rounds each range trip is not going to cut it......try 300 to 500 for starters, you'll see the difference in a couple of trips to the range. Shooting is a perishable skill and you lose muscle memory in short order.
