You should have found an idea before a co-founder. Your co-founder should complement the direction of the business and bring what's missing in your personal skill set.

First step now, is to figure our what each of your strengths is/are, and then create a list of potential ideas that spark each of you, then choose the one that (a) ignites you both, (b) has a realistic revenue model, and (c) can be built without the need for investor capital.

Make sure you choose something that actually has customers you can get to, not something that has revenue potential when you say "If we can just get 1% of the population..."

Lastly, enjoy the journey.