New as a co-founder - any advice?

Hello everyone I was offered a partnership with a new startup still working on its MVP. My participation will be my time and experience in my field. I will not invest any money from my side. I love the idea and the founder seems to be a smart guy. We agreed that I would work a few days a week with no salary until we get an investment or the company becomes self-sustained and that my equity share will be vested over the course of 18 months. I've started a business before, but this is my first time partnering with someone. Based on your experience, what are the things I should thoroughly discuss and pay great attention to entering this partnership, and make sure it is included in the shareholders' agreement?