Webbased , telnet or custom client ?

For a webbased game you should pick up atleast one of:

ASP.Net (C# or VB.Net works well)
PHP
JSP
or some other suitable scripting language.

+ a SQL dialect of your choice.

For a telnet based game (traditional MUD) you might want to look at some existing codebase such as RoM(written in C) (There are plenty of others aswell)
and simply modify the rooms,mobs,etc (no real programming required unless you need to add features), or write your own from scratch in whatever language you feel like.

For a custom client game (unnecessarily complex if you're only doing a text game) you can pick whatever programming language you want.

For PHP you can find documentation at http://www.php.net

For ASP.Net you could look at http://msdn.microsoft.com

There are plenty of tutorials avaliable for both aswell. (google is your friend here)