The first thing to know is to hire a legal professional familiar with startups because the lay people learn the law is by running into problems. It is also worth mentioning that studying "the law" can be a bit of playing house. By which I mean studying the law is often easier and less scary than developing a product that customers will by. It's more like school with the prospect of right and wrong answers (in theory) rather than a blank sheet of paper.
That's not to say that certain problem spaces don't have relevant law. But that law is relevant in defining the products and solutions and market validity...for example healthcare regulations when building Uber for cardiac surgery.
tl;dr Hire a legal professional and focus on product.
Good luck.