So you want to be a necromancer... Not a bad choice, but it's going to require a lot of work. Good career entry points
for this include: Occultists, Dabblers in Voodoo, Grave Diggers, Morticians, Inheritor of Scary Books Wrapped in Human Flesh.
First you're going to need some sort of Book of Evil (tm). We suggest: The Necronomicon, De Vermis Mysteriis, Unaussprechlichen Kulten, Cultes des Goules, the Pnakotic Manuscripts, or the Book of Dzyan. All of these are excellent sources of untold damnation. To purchase one of these tomes of horror you will need to go down to the little occult book shop down the way, with the withered old man running it who cackles insanely as he places the nightmarish book into your hands. You know the one. Warning: these occult book shops are known to disappear and be replaced by brick walls when people try to return, so make sure you get everything at once.
Once you have read the sorcerous tome and had all your sanity blasted away, you will next need to find corpses to fuel your army of the undead. Your best bet will be the local graveyard, but also don't forget the morgue, local battlefields, or killing your own! If you start getting any doubts about what you are doing (particularly the condemning of innocents souls to an eternity of pain and torment), rest assured that's only your conscience speaking.
At this point, you should have 1) A Book of Evil (tm), and 2) Legions of the Damned. All that's really left is some group of poor schmucks to terrorize. Absolutely perfect are villages of medieval peasants, but those are kind of hard to come by these days. If no medieval peasants are available, you should probably just terrorize the local town, as there is usually a castle or tower just outside it to unleash your armies of darkness from.
It is probably important to note that known side-effects for being a necromancer include: hair turning white, tomb rot, loss of skin, foul stench, eyes melting from their sockets, gibbering insanity, eternal damnation.