With so many different styles, sizes, features, and fits for packs, it can be difficult to select the right bag, but after researching, I have set
First decide which style of backpacker you want to be. Either you are a more traditional backpacker wanting luxuries, a more ultra-light backpacker going for a sub 10 lbs. base weight, a thru-hiker, or somewhere in between. Depending on your style of backpacking that will decide on which backpack will be better for you. For beginner backpackers, you generally will just stick to an internal frame backpack.
Second, I would decide how long you actually plan on being on the trail because this decides about how large of a backpack you would need (see Pack Capacity for more info). Next, I would look at how many luxuries are you wanting to haul or how many you feel you can go without. This is important because if you need the air mattress or the warmer sleeping, you will probably needmore space in your pack. Finally, I would go to an outdoor gear store to make sure you get a pack fitted to your body type, especially if this will be your first pack (see Pack Sizing for more info).
After deciding all of that, you should have the perfect pack for your travels.