A first-class lever has the input force on the opposite side of the fulcrum of the output force. You can calculate the mechanical advantage as you would any class of lever (see Levers). First-class levers can be used to redirect force and multiply it.
First-class levers can be found in everyday life, used as recreation and utilized as tools. See-saws, crowbars, and tool handles are all common examples of first-class levers.