Well you have internal frame, external frame, frameless and probably a few others I dont even know about. Try on alot of different ones with the weight you plan on carrying and walk around to feel how they fit. If you can find someone to properly fit you for a pack even better. IMO the pack should be the last thing you buy. If you buy the pack before the tent and sleeping bag you can wind up struggling to stuff it all into the pack. Do your reasearch do find whats most comfortable for you. If I were to splurge $$$ some bucks on one of the "Big 3" it would be sleeping bag/pad. A good night's sleep on the trail is priceless. Also you can save alot of weight with a good quality down bag. That being said most folks on this site lean toward "lightweight" backpacking (10lb base weight). I would recommend trying to go as light as is safely/comfortably possible for you. Ive found its tougher to make the change from high weight to lightweight as opposed to starting light from the jump. This site has some great stuff on it. Read all you can, hike as much as possible, and ask ALOT of questions.
Climb the Mountains and get their good tidings...
-John Muir