Ceiling fixtures can come in many different configurations, with various ways in which they are attached to the ceiling overhead. Most of the time however, a central threaded hollow rod holds a chandelier in place.
Ceiling fixtures commonly hang by two screws that attach the fixture base to the outlet box or to a mounting strap in the outlet box.
Before getting started make sure there is no electrical current flowing to the light fixture. This is done by flipping the circuit breaker in the circuit box.
Once this has been done, remove the metal plate which holds the wiring in place. This will allow you to get to the junction box. A metal box which houses connected electrical wires.
Unscrew the crossbar and wire nuts that are holding the rest of the fixture in place. This will leave a whole with only wires and the junction box remaining in place.