Ask HN: What Type of Business Filing?

I have been working on a project for a while and am planning on launching soon. The project is a b2b SaaS web application (vague I know, but I'll give more info on here when I launch). I guess the right thing to do before launching is to file for a business entity. I'm not planning on seeking investment or bringing on any other employees or founders anytime soon. What type of business should I file for? What other advice do you have?