Both Phat and JP gave you sound advise. It's all about where your going, what season, personal needs and knowing what you really need. Phat has a good list of what he carries. Just a suggestion and to get an Idea of what some of us do.
I use an REI UL60 I think it's about 3300 cu. in. I use a hammock and fly for shelter. 3lb sleeping bag with a closed cell foam pad. I don't carry extra cloths except 1 pr xtra socks. I tend to carry too much food but use a DIY alcy stove/windscreen with a Heine can for a pot to boil water. Meals are usually freeze dried. Add a few conviences like an Mp3 player/ GPS handheld, Petzel headlite and extra batteries. My pack weight with food and water is around 25 lbs. My next purchase is a digital scale so I can tweek extra weight.
You are probably getting the idea by now. Do a search and look at the different ways to go lightweight. It can be a constant learning experience. You don't want to suffer trying to carry lot's of weight. It's your choise though. My first trip bpacking was for 4 days. I carried way too much. When I returned home. I tossed 10 lbs of weight I didn't need. the next trip was much better.