If you have Google Ad Manager-specific questions, the best place to start is the Google Ad Manager Help Center.
The Help Center contains a number of articles in Manage mobile apps.
The Google Ad Manager Help Forum is also a useful resource for discussions, and where you can post your own questions.
If you have SDK-specific questions, start with a keyword search in the Google Mobile Ads Developers forum. When submitting a question to the forum, be sure to include "Ad Manager" in the subject line to ensure a speedy response from the Ad Manager support team.