1. If you are not sure what to build, have a bunch of fake features and see what people click on (have those fake links link to pages with "Under Construction"). Use Google Analytics of Mixpanel to track users' behavior. Learn to use customer behavior analytics.
2. See who is already fixing the problem you are solving, and productize their tactics.
Example: Startup books are super popular, so StartitUp made books into an interactive guide that easier to follow and do.