Depends on how you want to contribute.
As everyone else has said, Testers for PRs are always needed. Currently, there are about 301
open PR's. Of those only about 31 have received testing and the
RTC tag for successful tests.
Reading and cleaning up the Documentation is always a big project and is a good place to learn from and help.
You can always find more opportunities help on the volunteers site
https://volunteers.joomla.org/
PR = pull request, these are proposed code fixes
RTC = Ready to commit, these are patches that have been tested and could make it into a future version.
I'm currently asking for help in completing and validating the Code Standards implementation with PHPCS 2.7.x so developers can have access to the code style autofixers.
https://github.com/joomla/coding-standards/pull/109Additionally, there is lots of activity and help needed in Testing Joomla! 3.6.3 RC3 for release, completing the 3.7 and 4.0 versions of Joomla. We also have need for people to help with adding support for
PHP7.1 and
HHVM.
There is also a mailing group specific to the CMS development
https://groups.google.com/forum/#!forum/joomla-dev-cmsWith that being said, welcome to the community; and thanks for being willing to contribute.