The learning circle has a learning roadmap which is a suggested set of resources to work through to gain competency. There are also forum topics monitored by subject matter experts and available to anyone who wants to join in the conversation.
I have just started to work through the learning roadmap, and the first thing I have found is a set of videos on YouTube that are really useful. They are all short videos and they describe some of the key features of DOORS NG.
I can recommend the learning circle, for a good start in DOORS NG, but if you just want a taster, then the videos are a great resource. They show some of the differences between DOORS and DNG, how to migrate from DOORS Classic, and how to manage some of the requirements data schema.