How to begin for asp.net

start here:

http://www.asp.net/get-started

http://www.c-sharpcorner.com/Beginners/

ASP.NET master pages let you create a consistent layout for the pages in  application.
A single master page defines the look and feel and standard behavior that you want for all of the pages (or a group of pages) in application.
You can create individual content pages that contain the content you want to display.
When users request the content pages, they merge with the master page to produce output that combines the layout of the master page with the content from the content page.
CSS separate and rigidly specify the formatting from the text, then a single content (i.e., web page text) can consume many formatting styles without altering it's text contents at all.
There usually have 3 levels of CSS design:
1. apply format to an entire set of pages in the same time.
2.apply to one page by defining the CSS
3.apply to one particular element inside the page.
 

Hi
 Master Page concept is mainly for providing a consistence look to your web site. Means it will provide the constant look to other child web pages. You can use CSS to design those master pages. Mean using CSS you can give one rich lookup for your Master/or any child pages also.