Intended use, terrain, season are the initial criteria to determine.
Once those are decided, then the specifics of the shape, length, width and "thickness" of your feet determine which boot to buy. Most (all?) boot/shoe manufacturers have a physical "model" of a foot they use (called a last) in each length and approximate width when making boots.
The "last" they use usually differs from boot model to model. For example, i wear LaSportiva mountaineering boots, but only the "Nepal" lasted models. The other boot lasts simply do not fit me well - regardless if the length/width is "correct".
Most usually, with boots you get what you pay for, but not always (sometimes more, but usually less...).
Finally, shop for brands that are well known in hiking activities - not a tactical wannabe or a "Bass Pro Shop Special Offering (or Cabela's for that matter)".