The best bet is to take courses through Cognos. If you can't afford them (big bucks), then read the PDF files that come with Cognos.
Tackle one part of the software at a time. For instance start with Impromptu & learn it before jumping to another program. Go through the Cognos Discovering series of the manuals first. There is one for each part of the software.
PRACTISE PRACTISE PRACTISE! The manuals have the basics, but they are written an manuals and not as class-room books. Use the sample reports Cognos loads on your hard drive.
I have not used the book Nimmy has suggested, but I know it is written for beginners, so maybe start with it.
Good luck
Bruce
Bruce