You have come to the right place, then!
The Wonder source will be contributable in several areas:
interpreter.js: The core interpreter.cm.js: Standard library.vs.js: Predefined variables.
pm.js: The source for Delishusly.wonder.pegjs: The parser.
Be sure to check out the proposed features for some ideas.
Contributing will operate on a fork-and-pull system: fork the repo, make changes, and open a pull request. This prevents any merge conflicts and other nasty problems that nobody wants to deal with.
Familiarize yourself with how the interpreter works (including parsing!) before diving in. More resources will be added as time goes on.