You should google “PBR texturing for beginners” or “Basics of PBR texturing”… something along those lines.
There are plenty of tutorials on youtube and whatnot. As well as free software for texturing.
There are variety of options for how you gonna approach texturing. Hand-painted texture, or procedural, or baked.
You should also learn about creating normal maps if you want your texture to not look flat.
You could use Substance Painter / Substance Designer, or Quixel Mixer, or something else entirely.
Or, if you want stylized / cartoony texture you could, as mentioned already, just use photoshop and paint away.