Somewhere inside the service panel -- usually on the door -- there should be an index that shows what circuit breaker or fuse serves which areas of the house. The index helps you shut off power to the correct circuit before working on it.
Codes require service panels to be indexed, but installers sometimes don't bother to trace down each circuit. If there is no index, make one. To complete the index, work systematically to identify the circuit for every electrical outlet in the house. Be prepared to find some odd combinations. In older homes, a single circuit may travel through several floors. Or you may find an entire circuit serving only a couple of receptacles. Avoid attaching family members' names to the index; they might change rooms. Use more permanent directional designations such as "east bedroom" or "front hall."
Indexing is easier when working with a helper who can turn circuits on and off while you test the lights, receptacles, and appliances.