Contributing to Node.js can be done in a lot of different ways. You can start contributing to one of the working groups (like the Website, Diagnostics or Post-mortem group) or start teaching at Nodeschools, so your peers can learn from you.
You can also start working directly on the Node.js Core, whether you do it by answering questions, or actually adding code to the project.
The steps are:
#1 Fork the Project
#2 Setup your git Repository
#3 Build your Node.js Binary
#4 Write tests, Run the Tests
#5 Find Something to Work On
#6 Submitting your Solution