On the DHIS2 website there is a section with relevant links to different resources for how to get help.
There is also the Community of Practice. There are many experts in DHIS2 throughout the community so that is a very good resource to be aware of.
If you have any development-related questions, check the support channels on the Developer Portal.
Finally, DHIS2 is reachable over e-mail at [email protected], use this as a last resort.