Ask HN: Legalities with creating a startup?

Hi HN, I've been lurking and reading posts for a while, but finally decided tonight to make an account and my first post. A few days ago I came up with a marketable service, and have since decided to make a startup, but I'm unsure as to what I need to do legally in order to make this legitimate. I'm sure that part of this is surely relative to the state I live in, but are there some de facto tax forms or something I should be filling out? I'm very new to anything like this, and am willing to accept any sort of resources available. Thanks!