Ask HN: How do you split equity between founders

If you have 2 founders, do each one typically get 50%? What if a third joins afterward, how do you redistribute? What if an investor invests money later, do each founder give up equal amount of shares? I know it all depends on the particular case, I want to know what it is like typically in the startup scene. We have a couple friends but we have no experience on these things. Do we need to get a lawyer and sign agreement (that sounds odd between friends)? What if one of the founder "fade out" for whatever reason?