Extruder is running backwards?
You can either flip the connector for the motor around (i.e. plug it in backwards) or (if you are using Marlin firmware) look for the following line in configuration.h:  (using the Arduino editor open the Marlin file For your 3D Printer,  one of the tabs is labelled "configuration.h" click on that tab to bring it to the front for editing.  use the Edit, Find and put E0 in the find box, click find.  When you find the line below

#define INVERT_E0_DIR false


change false to true (or vice-versa). Note that if you go for the connector-flipping route, make sure that you only do this when the printer is turned off.