The Necromancer seems like not a good choice in D2R Ladder. His game style is hard to clearly defined when you first time played, but the players who take tons of time on testing his skills, they will absolutely enjoy his gameplay and realize how addictive build he is.

Though he is not a traditional DPS character, spells are important part of a Necromancer build but not as much important as Sorceress. On the other hand, when it comes to melee, Necromancer does not as good as the Barbarian as well. However, the Necromancer is good at some unique skills, such as poison, curses and bone spells. The best of all his abilities is to raise the dead to fight with him. The more enemies died, the more the Necromancer could enlarge to his undead army.

So what is the best build for Necromancer? First of all let's talk about the Skill Tree:

Curses: Amplify Damage and Iron Maiden each for one skill point

Summons: 20 skill points on Raise Skeleton, 20 on Skeleton Mastery, 1 on Clay Golem, 20 on Golem Mastery and 1 one Summon Resist

Poison and Bone: 20 points on Corpse Explosion

The key point is getting as many skeletons as possible as you can, meanwhile the Amplify Damage and Iron Maiden are also important. Corpse Explosion is the key skill and Clay Golem is your primary tool for dealing with bosses, such as Diablo and Baal. 

For the Mercenary, Act 2 Thorns is enough to help with the act bosses and allows you to use Amplify Damage for it as well.  

For gear, the runeword like the Lore helmet plus skills are good to use, the proper idea in your early game is just to look for basic survival gear and the best weapon you can find.

As you move into Nightmare, in the earliest you can have a four-socket weapon drop, Crystal Sword is good option, but it's also easy to actually get a high level items and six socket weapons from the quests.

Hell is the start of levels needed for Sunder Charm, since they require level 75 to use, you should have three core skills, Corpse Explosion, Skeleton Mastery and Raise Skeleton. 

On the D2R Ladder Items, using Enigma (Cuirass), Spirit Monarch, Harlequin Crest (Shako) and Grim Heart. More budget items like the Bone runeword can be used until you get Enigma with skills and resists plus.