Trial and error to start with
Reading books and most impotently mag help pages like in the PCA mag
Forums especially this one are probably the best way to learn, apart from doing a course
If you have friends that work in IT ask lots of questions, (not all at once they will get annoyed)
Some work places run free courses on word excel etc mine does.
Picking the most important bits out of software manuals
Software versions tend to lead on from the last edition to the next i.e psp7-psp8 so there is not so much new stuff to learn