The first resource to check is the documentation, make sure also to check the specific guides for the storage and virtualization solutions available in your cloud.

The existing discussions in the OpenNebula Forum and the archives of the former mailing lists (discontinued from 15th February 2015) are also a rich source of useful information.

Sign up in the OpenNebula forum and ask about the problem on there. Please be specific and provide information about your enviroment (OpenNebula version, hypervisor, operating system…).  The logs which OpenNebula generates can be a valuable resource when trying to diagnose a problem. You can easily activate them in $ONE_LOCATION/etc/oned.conf and $ONE_LOCATION/etc/defaultrc.

You can also find help in the #opennebula IRC channel on Freenode.