When @afre asked “if someone were interested . . . how would one get started . . .”, @briend 's answer considered what software to use and provided some nice guidelines on selecting colors.
My first thought was about “What to draw or paint to get started”, which covers a completely different aspect of getting started. So with the caveat that I’m perhaps an enthusiast, but many, many miles away from being an expert, I’d suggest starting out by sketching, shading, and coloring a sphere, perhaps using this approach:
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Set up a round sphere (maybe a tennis ball or baseball) on a plain background, the plainer the better. Make the surface a neutral color, or better yet give it a color that will reflect off the sphere. The sphere shouldn’t be reflective like a mirror - trying to sketch and color a shiny Christmas ornament would be introducing too many tasks that are extraneous to the goal of drawing, shading, and coloring a simple sphere.
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Add a light source off to one side, out of the proposed framing of the image but shining directly on the sphere. The light source can be whatever you want as long as it causes a fairly noticeable shadow.
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If it’s convenient, sketch directly from this simple “still life”. Otherwise take a photograph and use it as a reference photograph. Sketching directly from the “still life” is better.
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After sketching the outlines, add shading and color. This can be done at one go, or else by using LCH lightness or luminance blend to add the shading, and then LCH color blend on a separate layer to add the color. Using LCH blend modes will require using GIMP-2.9.
Any good watercolor or general “artist’s how to” book from the library will have a section on sketching and shading different shapes, and there are many tutorials on the web. But looking at the real thing and trying to make the image on the screen match the real thing seems to me to be a good way to get started. Too many people get stuck reading tutorials and never seem to pick up a brush and try actually making marks that can turn into a picture.