The first step in selecting the right backpack is to think about what kind of trips you'll be taking with it. If you're only intended to do short day hikes, you can get by with a smaller backpack. Though you'll still need to be able to fit some basic supplies, like water, a first aid kit, snacks and some overnight gear (though you might not be planning to be out overnight, if you get lost you'll be glad you packed it), a smaller, less technical backpack should work fine. If, however, you're planning multi-day back-country excursions, you'll need a backpack that's as up to the challenge as you are. Actually, it would be better if the backpack were more up to the challenge than you are since you'll be depending on it to make sure your trip goes smoothly.
You'll also want to make sure the pack you choose has comfortable straps, not just in terms of fit, but also padding. While your grandfather's old army pack may hold enough gear, old canvas straps are likely to cut into your shoulder and make you want to cut your trip short. When you go to try a backpack on (and yes, you should try packs on -- this is not shopping you can do completely online), check out the padding on the straps. You'll want enough padding to comfortably absorb and distribute the weight of your gear, but thick, bulky padding could end up being too hot and limit your range of motion.
Having your house fall apart would suck. Having your backpack fall apart when you're a long hike from civilization is pretty much like having your house fall apart, but on a smaller scale. Check the durability of any backpack you're considering. Look for seams that are double- or triple-sewed. Check out gear reviewers online to see how durable people who've actually used the backpack you're thinking about getting have found it. If you're going for a larger, frame backpack, check to make sure the frame is made of a strong metal, like aluminum. Also check all external parts. Any elastic or bungee cord parts should be thick. Straps should be thick nylon webbing and all clips should be heavy-duty plastic. Zippers should be protected from the elements. Most experts agree that nylon is the best material for a backpack because it resists tearing and is more durable than cotton or polyester.