Depending on your learning style and preferences, you can take two approaches to learning ROS:
Start the Tutorials - Dive in right away and start working with ROS.
Get an Overview - Read through this written overview of ROS and its capabilities.
To get an immediate start with a pre-prepared ROS virtual machine, please see here: http://www.clearpathrobotics.com/blog/how-to-guide-ros-101/
For more detailed information on the ROS framework, see the core ROS documentation.
There are three places to look for answers to your questions and problems. The first is this wiki. Try the Search feature located at the top right.
Check out the Support page for more information.
Wondering if there is already a ROS package out there that does what you need? The Browse Software tool also lets search for useful packages. You can also browse the list of public ROS repositories to look for groups doing similar work.
How to use this wiki
See navigating the wiki tutorial for more information.