One of the most important things to have fit you well on a hike is your boots. Your feet are taking the majority of the pounding, and if they give out, you're stuck.
Determine your Hiking Style
If you always do gentle hiking in city parks, you probably don't need heavy, serious hiking boots. The extra weight would just slow you down. But if you're doing lots of back-woods trekking and are carrying a heavy load, make sure you get boots with lots of support. You'll need the cushion to help your feet survive the weight they are carrying.
Determine the weather conditions you're likely to hit
Do you only tend to do day hikes on sunny days? Then you probably don't need the expense of all-weather boots that resist rain and snow and sleet. If you enjoy multi-day hikes or hikes in all sorts of weather, it'll be worth it to pay extra to have boots to keep your feet dry and warm.
Determine your ankle support needed
Casual hikers can do fine with low cut boots, i.e. boots that don't come up very high on your leg. Your ankles don't need support on flat walks, and there aren't going to be a lot of rocks trying to cut your legs. If you're going to be doing some climbing, though, you will appreciate both the extra support an the leg protection that a bit higher cuff will provide.
Try them out!
Don't just stick them on your feet and wiggle your toes. Make sure you're wearing the types of socks you'll really wear out hiking, and walk around the store! Do they slide? Do they feel uncomfortable? Make sure you really enjoy the way they fit before you rely on them.
Break them In
Like all shoes, hiking boots have to settle in to your foot shape and lose a bit of stiffness. Don't wear them out on a serious hike the first time you bring them home. Walk around the house with them, wear them out on errands. You'll get them comfortable and fitting well in a few days.
