Why would it be a good starting point? And why only some of them? What is the process behind objectively finding out which ones are good and which ones are bad?

It's a good starting point because the commandments were given by God. And without God, there is no objective moral standard. Everything, including your opinion on my point of view, is subjective and relative. Whatever one would want to call "good" or "evil" would just be a matter of opinion.