How much weight and bulk are you currently carrying? Usually people change out their pack after they have lightened their load so that you know what size(how many liters)and how much weight will your pack need to carry.
you might find a lot of useful info in the following articles videos and websites I have linked below, and welcome to BPL:
Mike Clelland(NOLs instructor and author, his books are very good),he has some greatfree videos on lightening up be sure to watch(his clothing system,the entire contents of his pack,water treatment and part 1 and 2 on the dinky stuff for ideas).This is pmags Lightweight Backpacking 101 and The Frugal Backpacker – The $300 Gear Challenge andBackpacking: A Beginners Primer .These are some other articles and videos for you to check out
Backpacking for Cheap: Gear for the Gearless
Backpacking Checklist (Gear List): 3-Season, 3-Day
Lightweight Backpacking 101: An Introductory Manual for Lightening Your Load Today (1st Edition – August 2001)
Cheap Ultralight Gear List
Oregon Field Guide: Ultralight Hiking
Lightweight Testimony: Lighter, Farther, Faster
Jamie Shortt talks about his progression and shows his gear list for each stage,
Lightweight Testimony: My Journey into Lightweight Backpacking
Clever Hiker’s whole website is very nice, but here are a couple to look at
CleverHiker Light weight Basic
CleverHiker Trail Skills
Andrew Skurka has a very good website with trips and gear lists and blog for you to check out
Andrew Skurka.com
here is a talk he gave at google
Ultimate Hiking Gear & Skills Clinic
His book is worth checking out also
The Ultimate Hiker’s Gear Guide: Tools & Techniques to Hit the Trail
You can get little repackaging containers (like the ones you see in Mike Clelland uses in his videos) many places including your local stores and pharmacy but here are some examples
USPLASTICS
USPLASTICS
Ultralight Designs
GossamerGear
MountainLaurelDesigns
You could make toothpaste dots like Mike C. shows or there are Archtek Toothpaste Tablets so only bring exactly what you need in a snack size bag for any given trip instead of a whole tube and these are good for little things like pills, toothpaste, tablets ect. so you are not packing a whole bottle(you can also get these small repackaging bags very cheap at most pharmacies or craft stores ect)