Many people say they are interested in helping out Haiku, but then get stuck trying to figure out what and where they can help out. This wiki page can serve as a source of potential things you can work on to help Haiku.
Haiku Archives https://github.com/haikuarchives
Hundreds of programs written for BeOS have been open sourced. Many of those programs have been gathered together and hosted at HaikuArchives on github.com. Many of these already have been recompiled and are running on Haiku. Many have several issues that need to be fixed before they'd be in working shape. For example, many have layout issues that need to be resolved.
Much work has been happening over at Haikuports. This is where recipes are written which are used by Haikuporter to generate many of the packages you'll find in HaikuDepot. Since many of the ports are still improving, there is always something to do here. You could work on writing new recipes, or take a look at the open issues there and see if you can solve any of open issues there.
If English isn't your native language, perhaps you'd be interested in helping support your native language on Haiku, or in Haiku applications.
Testing is another area many can help out in. You can test Haiku https://dev.haiku-os.org/, Haiku applicationshttps://github.com/haikuarchives, Haikuporterhttps://github.com/haikuports recipes and more. If you find things that seem broken, search the exisiting issues trackers and see if it's already been reported, and if not report it.
HaikuDepot is the central hub to find and install software. Only very few packages have been commented on or rated by users. Comments and ratings are very helpful when discovering software. Create an account with the HaikuDepot app and add your experiences. For more in-depth reviews of applications, create a forum post.
Coverity is a static analysis tool that is run on the Haiku source code every few months. The issues it detects are then reported. If you are good at C/C++ you can try fixing some of the issues it reports. Note that many are false failures, so we ask that you open a new Haiku trac ticket and attach your patches to those tickets for review.