The process depends on what the style you are looking to achieve. A painterly style will have different approaches to comic kind of style. You don't really need to specifically learn "digital art", because the principle of art is not gonna change. You only need to know about the controls and functions of your software and you are ready to go. Speaking of software, generally, if you have Photoshop, then just use it, good enough to do basic and advanced works, until/unless you find other software more convenient. Depending on what style you want to develop, you might need some brush plugins for PS because the ones comes with PS itself sometimes aren't that convenient in terms of efficiency. There are certain tricks that make digital painting so much more convenient, like layers and blending modes, masks and so on. It's worth looking at some Youtube video and see if those tool suits what you wanna do. I use MyPaint a lot, because I work on Linux and that software is so small and lightweight. You can also try Krita, a very powerful painting program, I'd say some of the functions are better than PS can offer in terms of painting. If you do watercolour or anything like that, you might want to check out Rebelle, it's a simulation based painting software, might replicate the water medium pretty well.