If you haven't already found them, there are lots of excellent articles for beginners and on gear selection listed in the left-hand column of http://www.backpacking.net/, the home page of this site.

Another good site for gear selection is Mark Verber's website. Lots of ideas for gear, including low-budget alternatives.

You stand a much better chance of having a comfortable pack if you take your gear and the equivalent in bulk/weight of a week's food, water and fuel when you try on your pack. And I would definitely forget about a sleeping bag compartment. In addition to the reasons listed above, it just adds unnecessary weight to carry!