Just start!
If your needs are as big as your head, then a lot.
You can probably just jump in and learn as you go.
None. Just look at the source code.
This one: http://libregraphicsproduction.com/.
None. You could theoretically learn how to drive a car without any lessons. You could teach yourself how to play a musical instrument just by messing with it.You could figure out how to set the clock on your microwave by hitting all of the buttons. You don't *need* to read any manuals, but it's usually the quickest route to productivity. :)