Consult Web sites for outdoor magazines and read their articles, product reviews and buying guides. Try various backpacks on for fit at a reputable outdoor store with a sales person willing to spend a bit of time with you. A salesperson will be helpful in suggesting packs and adjusting these for maximum comfort. Remember you can get great deals on outdoor gear online. Consider renting a pack and trying it out before purchasing it online.
Many outdoor stores offer a range of rentals.
Consider a waterproof pack with zipper pulls that can be opened with mittens if you plan to be out in rain or snow. Just because internal-frame packs are more popular doesn't mean they're the right pack for you. External-frame packs are cheaper, simpler in design, have more compartments, are easier to load and unload, can often carry heavier loads and allow for more circulation between the pack and your back. Don't forget essential gear you'll need in that pack, such as a water filter, water bottles, campstove and compass. Avoid backpacks which try to do too much - the more complicated the pack, the more things there are to snag, catch, or malfunction.