Open Wonderland is highly-extensible -- as a developer you may create new modules that extend its functionality using APIs for the Java(TM) Programming Language. Once complete, you may publish their modules on the Open Wonderland Warehouse site so that others in the community may use it.
You first should download and compile the source code for Open Wonderland. The Download, Configure, Build, and Run from the Source Code tutorial tells you how. The Working with Modules tutorials gets your familiar with installing new modules.
Next, you are ready to begin developing your own module. If you are a beginner to Open Wonderland and 3D graphics, Douglas Finnigan wrote two excellent tutorials for you: Part 1 (Basic Concepts) and Part 2 (Intro to Modules).
The Open Wonderland documentation wiki contains a rich set of developer tutorials. A good place to start is the 4-part tutorial series on developing a custom Cell in Open Wonderland (the primary means to extend its functionality). You can get started with Part 1 of that tutorial series.
As a developer, we'd recommend participating on the Open Wonderland discussion forums too; they are a great place to ask for help when you are stuck.