This is intended as a minimal checklist to help my friends get what they need to go backpacking with me!
I'm not 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 can leave behind more so than what you bring with you. Some backpackers will go to extremes to shed weight, like cutting off extra bits of their packs. Just remember that everything you bring with you, you have to carry.
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.
Always assume it is going to be colder and wetter than you think it is going to be.
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.