Should one need to learn php(or jsp) to create dynamic web-sites or just to use CMS ?.I am new to web-development...
If you just use the CMS without understanding how it works then you are limited by what the CMS will do out of the box or with plugins. You will have little scope for customisation, and not be able to fix things if anything breaks.
It really depends what you want to achieve. If a CMS does everything you need it to do, then you can probably get away with just using it out of the box.
(And now I will resist the urge to start a language war and just say that there are other languages beyond PHP and Java).