Always the best place to start off: http://wiki.ros.org/ROS/Tutorials
Summary of commands that needs to be remembered while working with ROS:
1. Navigating the ROS Filesystem
rospack find [package_name] Eg: rospack find roscpp
roscd [locationname[/subdir]] Eg: roscd roscpp
rosls [locationname[/subdir]] Eg: roscd roscpp/cmake
echo $ROS_PACKAGE_PATH Prints all package directories