You need to make sure your backpack can carry all the gear you want to take and that you can carry it (unless you're bringing a Sherpa on your trip, in which case: Why are you reading this?). Backpacks have different load capacities. You do not want to try to stuff 25 pounds worth of gear into a pack that's only designed to take 15. The knowledgeable staff at a reputable camping store can help you determine what gear you'll need and how much it will weigh (or you can weigh your gear at home).
Bigger hiking backpacks typically have two types of load support systems. Internal frame backpacks have a lightweight metal frame that runs inside the back. It's typically curved and molded to the backpacker's body. External frame backpacks have the metal frame on the outside, and they tend to not be molded to the backpacker's body. The external frame is more customizable. You can add or remove packs, or rearrange them to get a configuration that's comfortable to carry. Internal frame packs force you to use one setup, but because they're molded to the body, they tend to distribute weight well, too.
Different backpack straps also offer different types of load support. The last thing you want is a pack that puts all its weight on your shoulders. Your shoulders are weak (girlie man!) and will tire quickly. Instead, the pack should have a waist strap that distributes some of the pack weight to your hips, which are much stronger than your shoulders. A chest strap will also help transfer some of the weight from your shoulders to your torso. Cobra shoulder straps are also a good idea. It would be so badass if cobra straps were actually made of cobras, but cobra straps are wider and more padded at the top than the bottom. That means they can distribute weight across your shoulders and part of your chest without irritating your sides or restricting your arm movement.