So you want to contribute content? How far do you want to go?
Adding Encyclopedic Knowledge
Rosetta Code is a wiki, and encyclopedic content is one area where a wiki's indexing and cross-linking ability shines. Throughout Rosetta Code, you'll find terms unique to software development and computer science, along with mentions of compilers, interpreters and programming languages. You are encouraged to create and fill pages illuminating the arcana behind these. See the GCC page for an example of what a compiler might look like, for example. If you add new encyclopedic knowledge of your own, be sure to put it in the Encyclopedia category by adding [[Category:Encyclopedia]] to your article.
Start by reading up on adding a new programming example, and following those instructions. Wander around Rosetta Code and get a feel for the kinds of tasks Rosetta Code has to offer, and how code examples are normally presented. Be sure to read the Talk pages, where available, to learn a bit about the history of the page and the decisions that went into shaping it.
Once you've got a good idea of the programming side, and the kinds of tasks offered, consider adding a new programming task. If you are unfamiliar with wiki formatting, you can first test your submissions in the Sandbox. There are always directions Rosetta Code can expand.
Being a wiki, there will always be imperfect content on Rosetta Code. Such content typically gets flagged and finds its way over to one of the Maintenance subcategories.
Adding Organizational Infrastructure
Please confer with Short Circuit about adding things like templates and categories not dedicated to languages. If you've successfully participated in adding code, tasks and encyclopedic knowledge, and have demonstrated some degree of prowess in maintenance, he could certainly use your help maintaining and expanding the infrastructure of the site.