If you want to paint your own textures:
first, get a tablet. second, learn theory about light and how it interacts with materials, because that is how we percieve materials with our eyes. third, learn a bit about color theory, not much but the general idea about color affects us and how we percieve it. fourth, go on deviantart or pinterest or youtube and search for tutorials on how to paint certain material, how to paint hair, metal, stone etc. material studies for example.https://pinterest.com/search/pins/?q=material%20study%20tutorial&rs=typed&0=material|typed&1=study|typed&2=tutorial|typed
there are hundreds of those things. to goal is to relearn is how you percieve materials, and that each material interacts differently with light. from then on you simply search shit like "metal paint tutorial" etc, and learn from step for step paintings/drawings and do them yourself. there are also tutorials like "how to paint anime skin" etc, from those step for step tutorials you learn a lot about those materials. For example if your 3d object has skin, leather and metal, you want to learn how to paint those 3 materials.
once you have learned that, next thing is to learn the basics about 3d shaders, because shaders are basically faking the lighting interaction and does a lot of the job for you, often you only need to color 3d objects, and the shader creates the illusion of having lighting interacting with certain object, but you still need the material knowledge to set those settings and the shader right.
There are also websites like texture.com
or programs like substance painter, which have a library of realistic materials and textures, which you can use to paint your 3d object with, but it is still advised to have the background knowledge about materials.
Also there are a shitload of websites for 3d artists exchanging information and give tips, like polycount.com
The rest is a lot of research and a lot of reading and learning, good luck.