The web pages can be categorized in two sections.
1) Static Pages and
2) Dynamic Pages
Static pages are that which only contains plain text information. To build plain static web pages you can learn HTML and CSS.
Dynamic pages are the one which takes user interaction and changes its behavior on an event. Dynamic coding can be again categorized in two sections. 1) Codes that execute on users computer or in browser. Learn Javascript, Vbscript, Ajax, Jquery with HTML and CSS.
2) Codes that execute on servers. For that you can learn ASP or PHP.
Also learn SQL for managing Database on server.
w3schools.com provides good basic reference or you can buy books on each subject for the advance knowledge if you want to prepare by yourself.