Cool. I'd say that the most important things to figure out in detail are the Layers panel and the menus off of it (especially Multiply and Hue/Color/Saturation blending modes for changing the color of something without completely painting over it like Normal mode,) working with the various Select tools, the Paintbrush and Airbrush, the Levels popup under Colors for adjusting contrast and white/50% grey/black thresholds with more finesse than the one labeled Balance & Contrast, and the filter for Gaussian Blur - the last, I use all the time for creating outlines, mostly on text (use the text tool to type something, then duplicate the layer with the little button on the layers tab, check the tick box to keep transparency on the new layer, paint over it in the outline color with a big Pencil, then uncheck the box again and apply a small Gaussian blur. Shuffle the burred layer below the original and duplicate it until the outline is nice and solid, then turn off all the non-outline layers and pick "merge visible" off the Layers menu so you don't have a huge stack of layers to scroll past. This does mean you can't edit the text on the outline layer anymore, so I don't merge in the original text layer, to keep it as is.) the only critical keyboard shortcut I can think of is holding down shift and clicking to draw a straight line between the current pointer location and where you clicked last, which is really useful for matching curves by going in a bunch of short segments instead of trying to match them with the movement of the mouse. Aside from that, just play around. You can't really break anything as long as you make sure to always be working on a copy of your source image, so as not to risk saving over it with a crappy doodle where you were just fooling around.
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows