This is intended as a minimal checklist to help my friends get what they need to go backpacking with me!
I'm by no means an expert backpacker but in 2020 I spent about three and a half months hiking the Appalachian trail. This is what I consider necessary for a three season backpacking trip.
Some things to keep in mind. Backpacking is more about what you leave behind than what you bring with you. Some backpackers will go to extremes to shed weight, such as cutting off extra bits of their packs. Just remember that everything you bring with you, you carry. Every extra ounce of weight is an ounce you carry with you for the duration of your trip.
By far the most popular choice for backpacking is the the trusted tent. But more and more you are seeing people use alternatives. I sleep much better in a hammock then I do when I am sleeping on the ground. This will ultimately boil down to personal preference. Here is some information to help you decide.
There are two main types of tents you will likely be looking at. Freestanding and non freestanding tents. Freestanding tents are ones that come with poles that keep the tent propped up even when it isn't staked down. They can be picked up and moved around easily. Non-freestanding tents often use trekking poles to keep them supported. If you are already carrying trekking poles a non-freestanding tent could help save some weight.
Food often feels like the hardest part of planning a backpacking trip. What I will often do is just plan out my days of food.
When I first left for the trail, I had no idea what I was doing. I had a gallon plastic bag mostly full of trail-mix. A bunch of cliff bars. Homemade beef jerky and fruit leather. I hadn't brought a stove, and thought I would be heading home when I eventually ran out of food. I eventually realized that you could resupply on the trail relatively easily. I still didn't have my stove with me and found about cold soaking. Cold Soaking is when you soak food in water to make it easier to digest. Instead of cooking it. Certain foods do better for cold soaking than others. Instant mash potatoes, ramen noodles being two easy to get ones that soak quickly.
Always assume it is going to be colder and wetter than you think it is going to be.
In general I recommend running clothes personally. You want things that will dry quickly, and that you are comfortable in. I always start a running shirt and shorts, and then layer from there. Depending on the temperatures you may decide to bring more. Just remember that at night especially at elevation it can get cold. When I do layer, I usually do some combination of the following: base layer, fleece pullover, puffy jacket, rain jacket/pants.
Quality socks are worth it! Don't skimp on the socks! I've had good success with darn tough socks. I also generally recommend wool socks because it will wick moisture, and will also stay warm if they get wet, which is huge benefit.
I used to only carry a rain jacket. Then I was caught on a relatively warm day in rain and high wind. It got to the point were I couldn't stop shivering. I ended on the side of trail with my rain fly draped over some branches while I huddled in my sleeping bag underneath it. I ended up spending the rest of the day and night there. It ended up not being that bad, but taught me a lesson. Now I almost always carry a rain jacket and rain pants. Some people also really love pack umbrellas, although I've never tried one personally.
If you know me. You know I hike in sandals. I think you should hike in whatever you are most comfortable in. But what I typically recommend to others is a trail running shoe. I would also recommend a second pair of light weight camp shoes, such as crocs to give your feet a break when you are just hanging out.
It is often suggested that this be the last think you buy. You should first get everything else you are going to need and then purchase a backpack that can fit everything.