There are already too many good documentations online for inkscape. Simply click on Help - Inkscape Manual shall lead to the main reference. Although the icons are well placed and obvious with its function, but memorizing the shortcut keys for commonly used tools or operations is really the key to fast graphic editing. The shortcut key listing is available at http://inkscape.org/doc/keys046.html, which is also accessible via menu of Help - Keys and Mouse Reference.