For texture packs: photo shop is recommended. If you do not want to pay, you can use GIMP.
A tutorial for creating texture packs can be found
hereFor datapacks are no special programs required. You can use an online datapack generator to generate the base for your datapack.
A tutorial for creating datapacks can be found
hereCreating mods is very advanced. You need to know Java very well. It also depends on what platform you want to create the mod for. (Forge or Fabric). I say you need at least IntelliJ or Eclipse for creating a mod.
If you do know Java, but you think creating mods for Fabric or Forge is too difficult, you can try to create plugins. It is a bit easier, but you can still do much. The Spigot wiki has amazing tutorials. (Check it out
here!)