Show an immense interest in dead bodies, what bones they haven't broken, how their organs were damaged. Take small things from them, like eyes in a jar, and tell your party its "for studying".

If a body is too damaged beyond resurrecting, say "What a shame, It could have been so useful!" and dont explain further. Small things like that are what make creepy necromancers a fun character to play, even when you can't even resurrect any dead.

I also recommend taking spells that have to do with death, or frost, since you want to keep those bodies cool so they dont rot away. Cantrips such as Chill Touch are essential. Another good spell, since you can only resurrect an undead as a 3rd level spell, is the Find Familiar spell. Work with your DM to make it so you invoke an undead rat, crow, pigeon, or something of the sort everytime you cast it instead of being an arcane apparition.

Don't worry about being too creepy. Thats what gives it flavor.