I trenched a 12/2 nmwu from an existing plug outside which I tied onto, than to my shed where I tied it on to a motion light which also works fine. I than tied on an additional 12/2 nmd to the motion light and ran it to a handy box for a switch, that handy box also has another 12/2 nmd coming out of it to feed lights in the shed which aren't hooked up yet. When the joints are done in the box there is power everywhere and its all good but when I tie the joined wires in the handy box to a switch and turn it on the breaker always trips?

If the lights are not hooked up yet, what is the switch powering. Try this, run power to the switch and wire nut the leads do they are not connected. Of nothing happens you are OK. If the breaker trips, you have a short somewhere and the switch is connecting the fault .