Inkscape

Both Krita and Mypaint are bitmap applications. This is pretty standard for material emulation programmes, but it can be restrictive as it does not scale well. Vector drawings are infinitely scalable since the shapes that you draw in them are rendered, not mapped. Every time you load the image, or zoom in or out of it, every line and node is recalculated and re-drawn. While vector drawings are not the best way to digitally paint if you are seeking the most realistic experience possible, but it's essential for iconography, font design, or designing for several different mediums of different sizes.

Being vector drawing, the drawing process is substantially different than usual digital painting. It is slightly more calculated, but luckily Inkscape has ways to do all of those bothersome calculations after the fact if you prefer.

If you are more accustomed to freehand drawing, you can even trace over drawings by scanning your pencil sketches into your computer and importing the drawing into Inkscape.

You don't need an expensive scanner. In fact, a quick webcam snapshot is fine. Launch Inkscape and select File > Import and bring in the scan. When bringing in an image to trace, you are free to either embed or link; generally, it's safe to link. This way, when you're done tracing, you can delete the image and save on file size. The down side to linking is that if the picture for some reason moves or is deleted before you've finished tracing, the image will disappear from Inkscape.

There are many tools in Inkscape but the most common ones are the Draw Bezier tool, Draw Freehand tool, and the Edit Paths tool. These are located along the left side of the Inkscape window.

Your method is entirely up to you. One common drawing method is to do a rough trace of your image with the Bezier tool [Shift B] and then go in with you Edit Path [F2] tool and adjust the lines that you have drawn, providing them with curves and adjustments as needed.

Another useful method is to use the Draw Freehand [F6] tool to draw or trace wih either your mouse or tablet. This produces paths with far too many nodes, because Inkscape takes constant samples as you draw. Once your paths are finished, press [Control L] to Simplify Paths. This removes redundant nodes, having the effect of smoothing your lines and dramatically lessening the number of calculations that your computer must do in order to render your drawing.

Of course, the standard principles for fills and strokes apply. These are all accessed via the Fill and Stroke pallette [Control Shift F]. Like any vector illustration application, you can set colours for fill and strokes, and even apply stroke styles and effects.

When you have a finished product, you will probably want to adjust the sizes of your work so that the lines and different elements in your illustration scale as expected. You might also want to create a directory for your illustration, and copy the font that it uses into the same directory. No more lost fonts!
Inkscape is a vastly powerful application. There are a hundred tutorials online at http://screencasters.heathenx.org as well as books and blogs to learn more from. The more you use it, the more powerful it becomes, and you'll probably find it a lynchpin of your workflow.