Before you go to a lawyer have a conversation with your founders. You need to decide what your goals are. Are you looking to self-fund your venture? Are you going to go for angel funding or venture capital? Are you going to build something that is a natural fit with another larger company that might buy your company in the next couple of years? Is there a chance that you might have to spend a long time building something? What are the objectives of each founder? Are all founders committed to investing a full time effort? Do some founders provide greater value either in terms of technical knowledge or a network of potential customers? Do the founders have the resources to survive the startup stages?
I would have these discussions before you start with a formal agreement. At the outset everyone is excited about the journey ahead but it is better to have these discussions before there is money involved than after things either go big or go bad.