Digital painting is a broad field. In general, it includes everything about making paintings digitally. Usually, it also involves using specialized hardware such as graphics tablets. Digital painting is commonly applied (but not limited to) in Game Development, Film, and in general, the entertainment industry.
As for the start, it is a good idea to familiarize yourself with the field and learn from people who are already in the industry. ArtCafe podcast by Maciej Kuciara is one of the best places to start. I suggest you start from the first episodes. Additionally, The Collective Podcast by Ash Thorp might be handy. Last but not least Andrew Price (a.k.a Blender Guru) had a series of useful talks (in a podcast format) about the 3d modeling field on his channel. The majority of topics are applicable to digital art as well.
In order to get anywhere, I suggest using a project-based learning approach. Start from really small projects with a specific goal. This will steer your learning in the right direction. The projects could be small comics (1-2 pages, not more), illustrations for books, game assets, concept art (one piece is one project).
Even though some art can be made only with a mouse (for instance all Vector illustrations) for painting is better to have a device with a stylus. Here you have two options: tablet (such as iPad or Samsung book) or graphics tablet as an input device for PC. On the graphics tablet end, it is important to not fall into a trap of buying an expensive device. With that said I suggest you buy a simple tablet to learn the basics. Once you have learned how to draw you will know what you need from the device. To be more exact any tablet within $50 should do the trick. I personally used tablets from Huion and Wacom.
A good car does not make you a good driver. Fancy software does not make you a good painter. When it comes to software and skill one video comes to my mind. This video was made in 2006 and it is a speedpaint of a car in MSPaint (and by the look of it, made with a mouse). Even now, I am amused by how well done it is done.
For that matter, any piece of software will work. Which one to choose, depends on your personal needs and perspective of use. As with cheap tablets I suggest starting with free software (Krita, Gimp, or MyPaint). As for me, I would suggest going with Krita, since this software aims to be “painting” focused. And they do a good job. Once you get hang of it you can decide whether to go for an Adobe subscription or to by SAI or anything else.
Also, I suggest Krita, because you can learn Krita Secrets from me.
Good luck. And let the force be with you 🙂
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