To me handpainted textures aren't much different than just regular painting. If you're good at painting you'll be good at handpainted textures. So i'd just recommend taking some basic digital painting training like ctrl+paint. From there a program like Photoshop (good 3D painting but buggy), Mudbox(functionally better but…
@MMKH I found that too! ..by googling 'handpainted textures tutorial', As the OP should have done. If he did, he would've also found a link to an article on the Polycount Wiki full of fantastic information. Polycount is not your Google, @Sahar6262.