Light fixture is tripping the circuit breaker
If it trips the breaker right away (within a second or so) then you have a "dead short", which means one of the hot wires is touching a neutral wire or something else that's grounded. You will have to go through to double- and triple-check that the wiring is done properly and there are no stray hot wires.

If the breaker takes longer to trip (it can take up to 20 minutes in some situations) that indicates you have simply overloaded the circuit. Is there more wattage being drawn by the new fixture than the old one?