Ask HN: Advice on Building a Web App/SaaS.

Hey, fellow hackers, I'm a CS grad with a decent amount of programming knowledge and experience with Java/Python/PHP/JS and a few DBs - SQL and a few NoSQL stores. I want to build a service which allows for user sign up (form submission / CGI-stuff), data collection + storage + (DB stuff - NoSQL stores or MySQL) and displaying that data for each user (with some data viz). I've some decent background in UX/UI design, and have a clear idea of how I want the front-end to be in HTML5+JS. I've been looking at various MVC frameworks in Java to build something like this, but I've no experience building on them - Spring, Play, etc. And probably Hibernate for ORM. I do plan on having a REST-based API so that I can have other clients in the future. How should I architect it, and where do I begin? What would I need to educate myself on before embarking down this path. I seek your valuable advice HNers. Thanks in Anticipation.