So, I'm new to DnD and about to undertake my first campaign ever.   I'm dead set on playing a Wizard Necromancer, I've got all her personality traits and everything.  But I just don't know enough about the world to know exactly where my character would be from, based on the chosen Class.  That said, what places would she have learned necromancy and why?  I know why my character would want to be one, but I'm interested in also hearing what other reasonings you guys may have.

That depends on which world you're playing in.  Faerun?  Exandria?  Eberron?  Other?

Also depends on what that world thinks of necromancy, and whether your wizard gal is a regular wizard who studies the magical school known as necromancer, or if she's a capital-N Necromancer who raises the corpses of the innocent dead to serve her twisted, nefarious will.  If the former, much like Visa, anywhere wizards are sold.  If the latter...well.  That entails a talk with the DM to figure out how a card-carrying villainous capital-N Necromancer ends up as part of a (generally) heroic adventuring party.

Definitely Faerun.  Like I said before, I know the how, the what, the who and the why.   I'm just missing the where, as in where she could've found this knowledge.

Probably Thay, then. That’s where most Necromancers are from, I think (not an expert on Faerun).

Blackstaff Academy, in Waterdeep, also probably teaches courses in necromancy, but it might not be quite as respectably viewed.

Essentially any class can be justified as starting from nearly any location. For a Necromancer,

• Cities represent access to resources and corpses, while remote locations represent safety to pursue their training. 
• A Necromancer might learn from an academy, apprentice under a powerful mage, or self-study after having found an abandoned spellbook.
• In previous editions, there were cultures that had positive relationships with the dead, which included the Baelnorn, (Good-aligned liches).

It really is quite open ended, and a dynamic world has room for nearly any backstory. That said, I would probably enjoy reading about a character who grew up in war-torn region where the living were more dangerous than the undead. In order to protect themselves, they may have animated the remains of soldiers around their home, knowing ways of safely controlling or avoiding them as needed. When the whole world is dangerous, we often find comfort in surprising places.

"The Devil You Know"
"The Enemy of My Enemy is My Friend"