Ask HN: What one resource would have been helpful when you just started?

I am considering compiling a bunch of resources that a typical startup founder would found useful when they are just starting out. Something that is immediately actionable complete with frameworks, templates and resources about how to think about the MVP, how to prototype and get feedback, how to then go about building the product and how to work with various third parties.

What is one such resource/information/knowledge which you would have personally found really useful if only you had access to it at the time of starting, instead of finding it out later?