Ask HN: As startup founder, how do you know what people want?

Talk to your users! Before we started building our product, we spoke to hundreds of people in our target audience. We asked them their needs, their problems, and most importantly - what they wanted. We could zero in our solution to their problem by uncovering what they wanted most. So go talk to everyone you can about your idea and see what the audience you want to serve thinks about it.

try to get 10 customers and then email them and ask what they like/dislike about ur startup

You ask them.