Well, I’m biased but I would recommend a simple and dedicated “painting” tool like MyPaint to get started. It doesn’t have anywhere near all the features of GIMP and Krita, so it can keep you focused on painting and drawing. That spin sounded pretty good, didn’t it? If you really need to transform or resize something, you can right click on a layer and edit it in GIMP and it’ll automatically updated with your changes.
You can do quite a bit of painting without even messing around with layers and layer modes. Generally I use one layer unless I’m doing something that has a definite foreground, middle-ground and background, at which point I’ll use a layer for each. Start of by just picking ONE hue and only adjust your lightness and saturation. Some of the great painters used very “limited palettes” or almost monochromatic color schemes. Also, stay away from 100% purely saturated colors. This is just my opinion, but if you are trying to mimic most traditional physical painting styles you’ll want to severely mute your colors or they will literally scream out of the screen. In this regard I’m not really sure how useful Wide Gamut displays are, which is somewhat ironic.