If you are the founder, then just call yourself the CEO. If you are the one who makes the final decisions, you are the CEO.
If you are very young, VCs may eventually tell you that as part of the growth, the time will come for a "professional" CEO to replace you. Fine. In the meantime, you are the CEO.
For all the other co-founders, avoid C-level titles because they lead to title inflation at the wrong time. In a group of less than 8 people in a startup, the best structure I have seen is one CEO, and maybe some director-level titles (engineering, marketing). Sometimes a part-time CFO is justified. That's about it.