I'd reccomend you get the assets folder from the minecraft versions themselves.
If you need to find it it's in user/AppData/Roaming/.minecraft/versions. user here refers to your user's name in your pc and as for the assets once you open the folder versions find a minecraft version you've played and want the assets from, open it and right click on the version's jar file to open with winrar, then just take the assets folder out, you can also just extract the jar file if it's easier.
To then edit or make the textures I reccomend Paint.net or Gimp as these are free, but you could also use photoshop or any other alternative.
Then all you need to do once you've got your textures in the right place is place that edited assets folder into a new folder with the name of the pack along side a pack png and pack mcmeta, which will be your texture pack's icon and format. Hope this helps. And I also reccomend you check some tutorials in case I missed anything.

paint.net

go find some videos on how to do it on youtube