Why does my new light fixture trip the circuit breaker?
Most likely - you treated a switch loop (black hot to switch, white switched hot from switch) as "hot and neutral" (since they are black and white wires) so every time you flip the switch to on, you are shorting hot to neutral. So, the circuit breaker, as it should, trips.

At this point you need to sort out what wires go where so you can sort out what they should be doing. If you'd care to save yourself or the next guy some trouble, pick up some red electrical tape and put it on the ends of that white wire from the switch, if that proves to be the case.