I started on 6, and it's the one I still used. I've used 4, 5, and 7 for school projects, and tried the newer two, of course, but 6 is something of my baseline. I learned on it, and I like it best.
First, learn what you NEED to know for drawing. Things like brushes, layers, etc. Basic, basic things. Then, if you want to do graphic-editting, learn layer transparencies, layer blending properties, effects, masking, etc. (I recommend learning quickmasking regardless, but it's relatively easy to get the hang of) If you're doing advanced graphics, get the hang of non-raster layers and the pen tool, etc etc. Learn slices. If you're doing photo editting, learn how to mess with levels and the other edit tools. But it's easiest just to learn what you need, then extend it slowly to find out other things. It's like an easter egg hunt, don't be afraid to experiment. You'll learn it best that way.