As an open source project, Tox needs developers first and foremost. Graphic designers are very welcome too. Adventurous users can test and report bugs, translators will be necessary later on to translate stuff, and this wiki could always use another pair of hands!
Fork! GitHub makes it really easy to contribute to open source projects. Fork a copy of the Tox repo into your account, make some changes, and then submit a pull request to request to send your changes back to the main repository. Alternatively, you could work on a Tox Client as well.
Be sure to join the developer IRC channel to ask questions or just to keep up to date with the latest discussions around the project.
Need some ideas on what to do? Check out toxcore's issues page. There's always work to do to make Tox better!
Join the developer IRC channel.
There is an archived thread on /gd/: https://archive.foolz.us/gd/thread/91501/.
First and foremost is to try Tox out and report bugs. GitHub has an excellent Issues interface. Report bug or request a feature for Tox core, qTox client, µTox client, aTox client and Protox client. Don't be afraid to criticize anything that looks wrong — healthy debate makes Tox great!
Please read: How to Report Bugs Effectively.
Beside reporting bugs or requesting features, you could help improve qTox, aTox or Protox translations, or add a new language.
Edit this wiki, too! Even if you don't have content to add, you can rewrite for greater clarity, reformat for aesthetics, etc. For list of pages that can be edited look here.
If you have the resources to, you could run a DHT node. Nodes help new clients join the distributed network.