If you're just starting out, maybe size is less of an issue for you and cost is more of an issue? If you want a decent backpack that will be forgiving to a backpacking newbie, I think the Mountainsmith Boundary is a good choice. I just picked one up for winter backpacking and at 75L, 4lbs (after removing the top lid and a few straps), and $130 from Campmor, it's a darn decent backpack. If you're using an army surplus sleeping bag, I'd recommend a large volume pack like this.
However (to confuse matters, lol), for most of my own 3-season backpacking, with up to 4 days of food, I generally use a ULA Conduit, which is a 50 L pack and weighs 20 ounces, though that's a frameless pack (you have to use your sleeping pad internally as a frame), and really doesn't work that well when you're packing more than one set of clothes, a synthetic sleeping bag, or a tent...just not enough volume. For loads under about 25 pounds, however, it's a great pack.