You will mainly be using your mouse Left-Click to interact with the Game Interface and objects in the environment. You will also be able to display tooltips that give you information by resting the Cursor above the game elements and interface.
Spell and Item icons can be dragged by holding the Left Mouse Button and dropping the icon on Item Slots, Character Portraits or the Action Bar. Alternatively you can use your mouse Right-Click as a contextual shortcut.
In the Environment you can move your mouse while holding the Right Mouse Button to move the camera freely to look around. When you release the button the perspective will snap back to its default orientation.
Many actions in the game are linked to keyboard shortcuts. You can view and customize your shortcuts in the Options Menu under the Hotkeys Tab.