Lots of info for the beginning backpacker about backpack selection, gear and other aspects of backpacking is on the home page of this site, left-hand column.

One thing not mentioned (unless I missed it) in the excellent advice already given here is that the pack needs to support the weight you expect to carry. That's of course one big reason (other than volume) for getting your pack last. Manufacturers' websites will state the maximum weight the pack can carry. Sometimes this number is a bit optimistic, which is why it's important to try on the pack with all your gear inside, including water and the equivalent of a week's food. There's a lot of difference in the feel of a pack with weights inside and one properly loaded with real gear. Don't just try the pack on, but hike around the store (or your house if you order through the internet) for several hours fully loaded.

One thing to realize--pack fit is almost as individual as shoe fit. What is comfortable for someone else may be painful for you. That's why many of us won't recommend a certain brand over another. I can't recommend the one I use and love because it has been discontinued. Its closest equivalent is the Six Moon Designs Starlite, with the "optional" (mandatory, IMHO) stays added. This is not a pack for loads much over 30 lbs., though.

Unless you plan to drag your pack around on the ground or toss it off a cliff, there's no reason to get one of those "bomber" packs. All a really heavy pack will do is weigh you down. You do want a good quality pack, though, so you probably want to avoid the cheap packs sold in big-box discount stores. You really should find a local outfitter with experience in fitting packs, even if you have to drive a long way to do it.