Mistress of Necromancy, Death, and Undead

[emoplato]

Should I have Greater Spell Focus (Necromancy) and Spell Focus (Necromancy) if I am an wizard?

Those are decent but you could do better.
Here is a handbook for general wizards.
Dictum Mortuum's Handbooks: The Wizard's Handbook Part I: Attributes, Races and Class Features
I suggest you get a way to heal undead. Lord of Uttercold doesn't take up a higher level so it is free for you, skeletons are immune to cold.
http://dictummortuum.blogspot.com/2011/09/wizards-handbook-part-one-attributes.htmlhttp://community.wizards.com/go/thr...2/Treantmonks_guide_to_Necromancy:_Gods_Tools
Here is a break down of some wizard Necromancy spells.