How Websites Work

Hyper Text Markup Language (HTML) And Cascading Style Sheets (CSS)

HTML is a markup language which is a set of markup tags and text and it is the main markup language for displaying the web page in a web browser.A Web browser interprets these tags to decide how to format the text onto the screen.
CSS is a style sheet language used for styling html pages.

Read some tutorial for learning html and css :wwww3schools.com/html/default.asp
www.w3schools.com/css

where to write html or css codes

You can write this in your text editor but To get started Try some code editor i used Notepad++


where to put html or css files

on a web server, web server deliver web pages on the request to browser/clients using the Hypertext Transfer Protocol (HTTP)


read here to know more about webserver and httphttp://en.wikipedia.org/wiki/Web_server
http://en.wikipedia.org/wiki/Http


Web browsers request pages from web servers by using a Uniform Resource Locator (URL).

read here to know more about URL
http://en.wikipedia.org/wiki/Uniform_resource_locator