Welcome to the forum.
Is this your first time going backpacking?
If you haven't visited the main site (backpacking.net) give it a look - on the left column there's a section talking about gear lists. One of them is an 18 lb 3 day pack.
I'd also suggest looking at forecasts for the area you're going to see what the estimated night temps will be like, and checking out some of the local sporting good places to see what you can rent that's suitable. If you haven't backpacked before, having a little experience before you start buying gear will help you make more informed choices without ending up with a closet full of packs and sleeping bags. Gear can be quite expensive (or cheap, depending on your needs). There really are no pat answers because needs differ from one person to the next, not to mention all the considerations for the weather and season.
Another good site to look at when contemplating gear choices:
http://www.verber.com/mark/outdoors/gear/index.html Lots of details on different kinds of gear, clothing, etc.