MongoDB-user and MongoDB-dev mailing lists are good resources for asking questions or learning from others’ experiences. MongoDB-user is for users building applications with MongoDB, whereas MongoDB-dev is generally for users contributing to the database or building third party tools.
StackOverflow is another good resource. Post your MongoDB questions on StackOverflow, tagging your questions with mongodb. Also, be sure to check out the #mongodb IRC channel on Freenode.
Of course, if you can answer someone else’s question on any of these sites, feel free to jump in.