First, establish the use for the backpack. Will you be wearing it all day, in all kinds of weather, while walking 8 or 12 hours through nature? Or will you be putting it in the luggage compartment of buses or trains? Will you be using it in a city where you need to worry about pickpockets or bag-slicers? Does it matter that it fits the rules for carryon sizes on airplanes? For which airlines?
Second, establish the volume you want. You can get anywhere from 20 to 90 litres with backpack straps on it. I can fit all I need for an indefinite trip into 45L with the help of packing cubes, but I don't bring a hair straightener, 5 spare pairs of shoes, a bathrobe and so on. If you already own some suitcases or backpacks, you can do some trial packing to work out what volume you need. Compression sacks will help make things like sleeping bags or sweaters smaller, but remember they don't reduce weight.
Third, establish a budget. If you think you don't have an upper limit then you haven't yet looked at what the most expensive backpacks cost. You can spend a LOT of money on a high end Tom Bihn, for example, but do so because the slashproof fabric or waterproof zippers meet your needs. Or rule it out because there's no internal frame to help you carry that all day long on a trail.
Finally, start reading reviews looking for comments about adjustability or reliability or waterproofness or whatever. It's entirely likely some of the items in the reviews won't matter to you. That's why you have to think long and hard about how you're going to use it.