Please teach me how to create a new file such as phtml, php, css and also update those files.
It has been 2 weeks since i started to use Magento so i am very beginner...
study online documentation for the same. Now there are nomber of resource & tutorial available.. http://magento.com/help/documentation
For adding js and css you can refer to
But for adding php file you will have to go through magento documentation.