Launching a startup business with a friend can be an exciting and rewarding experience. However, I suggest you take a moment and really think about what the potential consequences might be. Starting a business is tough and having a friend involved will only make it more difficult if things go sour.
Most importantly, your friendship should not be a substitute for a proper set of bylaws and other corporate documents. You and your friend will appreciate these documents because, if properly drafted, they will provide you both with clarity regarding what will happen if certain events occur.