If the circuit breaker trips immediately when you switch on the lights then you have a short in
a. the wires between the switch and the fixtures
b. one of the fixtures or
c. one of the lamps (lightbulbs)
c. is the easiest to test, so do that first -- remove all the lamps and see if the switch still trips the breaker.
If it's not that, then you'll have to start checking deeper; separate some of the fixtures from the others to narrow down where the problem could be, etc. I once saw a house where mice had eaten all the plastic insulation off the electric wires in the attic, leaving the copper conductors free to touch and short out.
ChrisV
October 2007