Anyone can be a cofounder. Most often its friends, or friends of friends, but sometimes its an interested professor(MIT has a long history of this) or random people in your study group that show an aptitude in the field. I would strongly advise you to approach everyone with your idea, explaining what you hope to achieve and the possible setbacks/technical hurdles to come. Ask everyone you know if they know anyone who does something similar. If they come back at you with a few good ideas, or approach the problem from an angle you haven't thought of, then you have a candidate. Eventually you'll find someone as enthusiastic and interested in the area as you, which is perhaps the most important trait, and its a good indication of the type of person who will stick it out and help you change the world.