If you have Google Ad Manager-specific questions, the best place to start is the Ad Manager Help Center for your product:
Under the Mobile topic in these help centers are Getting Started guides for mobile ad serving. The Ad Manager Help forum is also a useful resource for discussions, and where you can post your own questions.
If you have any SDK-specific questions, the best place to start is to search for keywords related to your inquiry on the Google Mobile Ads Developers forum. When submitting a question to the forum, be sure to include "DoubleClick" in the subject line to ensure a speedy response from the Ad Manager support team.