This page is a simple guide to help develop for mediawiki (e.g. javascript or lua modules).
Developing javascript gadgets
There are a couple of things to get acquainted with. First of all knowledge of javascript is necessary. One also needs to get acquainted with the mediawiki library. One good resource to learn basic libraries is the course in codeacademy that covers mediawiki and wikipedia api/library found here:
Developing lua modules
As with javascript, developing scribunto modules requires knowledge of lua. Short lessons can be found in Wikibooks:Lua. Also try going through the Lua templating/Getting started to get acquainted with more complex uses.