How to develop product vision for startup idea?

I have a startup idea I've been thinking about for some time. I have a decent (I think) idea of what I want the product to do (help people invest better using fundamental analysis), and what the mission is, but when it comes to thinking of what the actual product is, or what it looks like (what the UI/UX would be), it almost feels like I have "writers block" and get stuck.

Does anyone have advice on how to get past this stage? Should I be thinking about how to build product vision / skills? Do I need to talk to actual potential users? Should I try doing spec work with a designer? Apologies for the n00bish Qs here, but any advice / insight appreciated.