The easiest sort of witch for research is one that you create yourself. Yes, you do need a coherent scheme to your magic (and I'll get to that later), but in the meantime, you can choose to have magic behave any way you'd like it to.
Is it generic? Or an aptitude that some people have but others don't? Or maybe the possibility exists within everyone.
Is it done with runes, charms, incantations, or only with the power of the mind?
How did this magic come about? Were the witch hunts of long ago actual witch hunts? Did Merlin (or some other famous witch) exist?
These are all questions you will have to ask yourself when the time comes. My post summing up this short series on witches and witchcraft will help to nudge you in the right direction (if you don't already have some ideas floating around in your head) but in the meantime, why not take a look at some of the books of these popular authors:
Wicked Deeds on a Winter's Night
Cheyenne McCray, Magic series.
Fur for All
Wolf at the Door