I worked my way through college as an electrician so I know enough to answer this question.

If you are a novice, the best way to deal with this is to trip the circuit breaker. The reason is that some ceiling fixtures have the hot wire coming into the fixture, then run a wire down to the switch. You could still end up handling a hot wire at the fixture.
Some fixtures are wired with the hot wire coming into the switch, then a wire up and back to the switch. This would be safer to work with at the fixture, but it's just easier to kill the CB.
If you are wondering if wires are hot, you can always buy an inexpensive amp-meter, set it to read AC current then test wires.
With a new fixture, it's important that you ground the fixture itself. Usually there is a little green screw. You can connect a wire from this to the ground wire (bare wire) in the ceiling electrical box. Feel free to ask more questions.