Back when I was in high school, I did a week long backpacking trip on the Appalachian Trail (AT). It was not terribly long, but a good challenge for me at the time. And it was just enough to whet my appetite for backpacking in the future.
I had never done anything like it before, and had to prepare from scratch. I used a large part of my allowance for the year to buy boots and pay for the trip. I borrowed a backpack from a friend, who had done the trip the year before. She had raved about how much fun it was, and encouraged me to go. I had my own sleeping bag - all kids do, right? In retrospect, I should have tried to borrow a proper one. The rest of my gear consisted of shorts, a pair of jeans and a couple of t-shirts, pretty simple.