How can I replace my light fixture with a ceiling fan?
Two part answer:

The North American wiring coloring standard is black for hot and white for neutral. But unless you've performed the wiring installation yourself, you can never assume black is hot. You should obtain a house wiring tester and test which wire is hot and which is neutral. Consult your fan instructions and connect the hot wire to the hot wire and neutral wire to neutral wire (assuming end or parallel installation).

For the weight of the fan itself, it is best to determine if the ceiling box (if you have one) is attached to a structural member of the ceiling with a lag bolt. If it isn't, consider adding a new ceiling fan box that would attached to a structural member. You would have to patch some drywall but it's safer than having the fan fall down at an unexpected time.