This determines if you should purchase a left or right handed shotgun; if your right eye is dominant, buy a right-handed shotgun. Some shotguns can be used by both right and left "eyed" people. Generally, you should expect to spend: $100 USD on a break-open single shot $200-$500 on a pump-action shotgun $500-$1000 on a semi-auto shotgun $1000+ on a worthwhile double-barreled shotgun Pick a spot on the wall Close your eyes Point the shotgun at that point on the wall Open your eyes The shotgun should have come to your shoulder easily. The stock should not be too short for you. Ask the shopkeeper for advice on this matter. You should not have to deflect your head very much from it's natural upright position to place your eye correctly over the barrel; assuming this position should be natural. The shotgun should be pointing at the spot on the wall when you open your eyes.

Summary:
Find your dominant eye, generally the same as your dominant hand. Form a budget; depending upon how much you are willing to spend, you may be able to rule out entire types of shotguns. Go to a gun store with a large selection of shotguns Find a shotgun that fits you: Inspect this shotgun for damage or rust. If you like the shotgun, it's within your budget, it shoulders naturally, and it's mechanically sound, then you should buy the shotgun!