Necromancy is not what some think, thanks to books, video games, movies, and some tall tales. It is not the reanimation of dead bodies, nor is it the control of the dead. Necromancy is the belief that one is contacting the spirits of the dead, rather than those of angels, demons, deities, and the like.
Many people who practice necromancy will advise that the safest place to start is with honoring one's ancestors -- maybe even with an ancestral altar, and known passed loved ones are said to respond most readily.
Considering it has the suffix "mancy," it is clear by name to either be a form of divination (seeking guidance from those who have passed) or it is using divination to contact those who have passed. The more common use of the suffix is that the prefix defines what method the divination is, such as cartomancy being divination using cards. Necromancy, then (and this is coming from a non-practitioner) is using information garnered from those who have passed, via ritual, meditation, etc., as a source of divination.
Some say that those who have passed can give great insight not only from their own lives, but can also see what is going on, and offer advice based on a very wide external perspective.
But there are warnings about abusing contacts, as it is said that even beyond this life, there is plenty to do, before progressing further on -- whatever that may be. Calling too often and for frivolous reasons can (supposedly) upset or anger the spirits of the dead.
Hopefully someone can suggest some decent reference materials.