That's really up to you and can be as stereotypical or out of the norm as you like. Maybe you're just like a spooky scientist that thinks undead animation is super interesting. Maybe you hate all life and want to end it. Maybe you think deep down you're really a good guy, but maaaan do you love having automatons to boss around.

A few posts I'll link here are great for formulating roles and possibly understanding CF level role play in general.

http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=5&topic_id=2584&mesg_id=2584&page=

http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=5&topic_id=402&mesg_id=402&page=

I just scanned, but have read in the past. I believe the first is mostly in regards to back story while the second is mostly in regards to how your character actually acts in general interaction.

As you play, be aware that some people treat this game more as a Call of Duty in text than they do a role playing game. Some of us though are intensely focused on the role play. You also have the gamut in between the two extremes. So don't get disheartened if some dude comes along, curb stomps you, and then doesn't pay any mind to anything you say: silently walks away.

Personally, for a first character, I would make your guy very focused on exploration/information. Like for a necromancer you might be on the hunt to find and research all areas of "desecration" or something. Anywhere you find channels of unholy energy or something. (EDIT: Then proceed to ask people about graveyards or whatever you think might help your NEXT necromancer succeed if you knew it very well. I think graveyards is one, but I don't do necros) You can feel free to make up some aspects even. I've made up alternate death theories and things that my chars believe which don't actually fit the reality of Thera. Just like real life though: some people believe in spaghetti monsters and some don't! Your character doesn't necessarily know for sure about anything.