First off, congratulations on taking the leap to founderhood. In the early stages of a startup, roles are often complicated. You may bring the idea and vision but your other co-founder should also be able to contribute and shape the vision. The product often gets put in the middle of power struggles between founders. Work with your co-founder and share the vision you have. Allow him todo the same and bring both of your visions together. It is important that you do this with an open mind. Your vision needs to be flexible. If not these type of conversations will not be productive and tensions with your co-founder will become more announced.

Your job as CEO is not to veto ideas from co-founders but to bring clarity to the vision. Mark Suster uses the analogy of the CEO being the Chief Psychologist (Check out his post on this, "My Life as a CEO (and VC): Chief Psychologist").

Also remember that the decision maker will ultimately be your customers. When pre-traction we often look to ourselves as founders for the answers. As you gain customers their interest and lack of interest should be a big factor that ways into decisions.

To help with product decisions sit down with your co-founder and develop a product roadmap. This will help bring the high level vision down to execution and tactics.

A book I emphatically love and recommend for first time founders "The Founder's Dilemmas: Anticipating and Avoiding the Pitfalls That Can Sink a Startup".

I hope this helps. I would love to have a call with you to discuss this in more detail. Best of luck to you and your startup.