Changing Drupal theme completely I am a newbie, I am in the middle of the website which was already created using the bartik theme and I want to change the theme completely like the way Main-menu looks and adding the slideshow in the middle of the page. I really don't understand how to do it. Any suggestions? Thanks.
I would recommend starting with the "Stark" Drupal theme.  It's the minimal theme,  easier to customize.

You are also going to need to read about some basic Drupal concepts.  Views, blocks, page templating.

Your question is far too "big" to be answered in this forum.  You will have to read the documentation at drupal.org