Where did you place your endstop for the x-axis? On the motor side or on the other side? Try to control your printer from the print panel in repetier or with pronterface/printrun. Going minus should be towards the endstops, going plus should be away from the endstops. If the controls do the opposite than you can fix this in 2 ways. First is quick and dirty: just reverse the connector of the steppermotor on the ramps board. Second solution is adjusting the firmware. Don't know the firmware you're using, so I can't give you any advice on that.
Did you confige the maximum size correctly in the firmware? Maybe used a "," instead of a "." in the dimension?
Did you configure the amount of steps for the x-axis motor correctly? It depends on the kind of motor, belt and pulley you use.
Hope this can guide you towards a solution.