Check with the help(F1) which is very useful for your reference.
Check with the Qlikview reference manual.
You can search this forum for ideas to get expertise.
You can have a start from here http://community.qlik.com/docs/DOC-1986
Go through this videos.
Download reference manual from the community, it contains all the explanation about Qlikview charts, expressions, Built in functions etc.
it will helpful
and Reference Manual will be more helpful for you to understand concepts