(This is for if you don't know how to and wont have anything advanced and might have some extra unneeded stuff such as: extra files, extra blockstates and models, and every vanilla texture in the version you want)

The easiest way to start if you don't know anything is to make a folder and put a pack.png (the cover, not required), and the pack.mcmeta. Inside the pack.mcmeta should look like this:

---

{
"pack": {
"pack_format": 6,
"description": "Description"
}

---

The pack format number is the version of mc (1.16=6,1.15=5,etc)
And the description is, well, the description.

---

Once you are done with that, you can go to your versions folder (User>AppData>Roaming>.minecraft>versions). Here, you go into the folder of the version you want (if its not there, you have to launch minecraft on that version) and then grap the ".jar" file. Then it is best to make a new folder, put it in, extract/unzip, then you can take the assets folder out, delete all the rest, and put the assets folder into your folder with the pack.mcmeta. The assets will have all textures, models, and blockstates for you to work with. If you want to work with optifine or more advanced stuff, its best to watch videos to do so.

---

For it to work, the pack.mcmeta should be there with everything inside typed correctly, and you can go to your texturepacks folder which you can get to in either .minecraft (User>AppData>Roaming>.minecraft>recourcepacks) or by clicking "open pack folder" in minecraft after clicking "options" then "texture packs" and put either the folder there or you can highlight everything inside (assets, pack.png, pack.mcmeta) and zip it up to publish to PMC or to use on your own.