Hi-
Good question, maybe the most important one. Some quick advice...
No matter what skills/roles you want a co-founder to fill, don't commit to anyone until you've had a chance to work together. if you have a former colleague in mind, that's a leg up but still proceed carefully. Working together on a start-up is like nothing else.
Look for someone who can get the passion you have, maybe even more.
Look for someone who can afford to get paid very little for awhile. You don't want a co-founder who has one eye on the door if day-to-day survival cash is an issue. Nothing wrong with that but that's just a fact of life.

Set-up a call and we can dig a little deeper.