I hope that you are going with someone with more experience. A 5 day trek for someone who has no experience is quite an undertaking. IIRC you are a youngster and probably in good shape so the physical demands shouldn't be too problematic. Carry more food than you think you will need - somewhere around day 2 you will start consuming 1.5 to 2 times as much as usual. Carry less gear than you want to. Everything you carry is a burden! Don't skimp on essentials - multiple socks makes more sense than an extra shirt. An ultralight umbrella is a good way to save weight - if it replaces a heavy poncho or rain-suit.

Pack clothes for how warm you will be while walking vigorously. If you aren't walking you will be in your sleeping bag. Don't pack the type of clothes you would for walking around town while poking in and out of shops and cafe's. You will be exercising. If you carry a heavier pack you will be working harder and be even warmer. A lightweight pair of long underwear will be more useful to you than a heavy sweater.

As for the pack. Remember it only carries stuff it has no other functional use. Once you setup camp for the night it is useless. Make it as light and simple as you can without sacrificing carry comfort.

I used to carry a medium alice - now I am making my own. Going from 7 pounds to under .5 pounds. That would allow me to carry 6 more pounds of gear or walk 6 pounds lighter. I am actually shaving weight everywhere. For example my 4 pound tent has been traded for 14 ounce tarp.