There are a range of ways to contributing to Babylon.js from finding and reprorting bugs, adding to the documentation, creating new materials and procedural textures for their libraries to developing an extension or a new feature for the core.
How to Start Contributing to Babylon.js
Setup Visual Studio to Contribute to Babylon.js
Approved Naming Conventions
Creating Material for the Materials Library
Creating a Procedural Texture for the Procedural Textures Library