Art is like singing - training helps, but isn't required. It's not about winning competitions or even having people like what you're doing. It's about expressing and releasing your emotions, catharsis if you will. The good thing about online doodling is that the software helps out even if you've never doodled before and don't have the skills, allowing you to produce pieces that you wouldn't be able to do with 3d supplies. You can read below about the four apps on this page, but basically the first two are simple doodling ones where you just drag your mouse around the screen and see what happens. The last one is a photo editor, and the third, Sumo Paint, is what all of the below ideas are about.
Originally the word "mandala" meant "a geometric figure representing the universe in Hindu and Buddhist symbolism." Nowadays it's used more broadly, to mean a symmetrical round design. Mandalas are easy to draw with the paintbrush tool, just check on the Symmetry option and play with brushes and the opacity settings. If you've never done it before and aren't sure where to start, a good approach is starting with bigger shapes and less contrast colors (e.g. drawing a light pink circle on a light blue background), and then proceeding with smaller shapes in more contrast colors (e.g. putting smaller dark red circles around your pink circle, and finishing up by putting white dots in the middle of each dark red circle). You can also use the stained glass technique (below) for mandalas, first drawing symmetrical designs with a brush, and then coloring spaces in between them with the bucket tool. Symmetrical designs are calming to human eye because they represent order and logic, rather than chaos.
Very simple technique: you draw lines with the line tool, and then fill the spaces between them with color (using the bucket tool). A good place to start can be to just draw a few straight lines in random directions across the screen and color the spaces in between, to get the hang of it, before moving on to actually drawing something. You can make the lines thicker, thinner, or change their color. When using bucket tool, you might want to change the tolerance setting to about 80% - that fills the space neatly, right to your lines. You can always undo, or paint the same space again with a different color. Once you've colored everything, you can adjust all of your colors at once through the options under the Adjust menu, for example the Hue/Saturation option. Great way to make an avatar by the way :)
The most flexible technique - just pick a brush, pick a color, and see what happens. Do play with shape/size/strength settings on brushes, selection is large and sparks creativity even if you didn't have any particular goal in mind. There have been studies showing that colors influence our moods, so simply messing with them for a while, even without the intent to create anything worth saving, is still good for mental health. Naming what you created is also good for both mental health and creativity: you can name your abstract color clouds anything you like, from "Jazz I have playing on background" to "My last night's nightmare".