Fool around in Photoshop and test out all of the tools, filters, etc.
I learned my way around the programs just following youtube tutorials as a starting base, and then branching into text tutorials, and so forth.
The more time you spend in something, the more you'll understand it. (Obviously)
Also don`t expect just to become good at this, for most people it takes months if not years to become intermediate let alone a professional . Tutorials , lessons and such are a good start ,but everything is in practice .