The role of an investor is one of intellectual curiosity and always looking for interesting trends. Developing these opportunities can evolve from an industry to a trend within that industry. You can know the e-commerce is only 16% of all retail. That's an industry with a lot of growth potential. Within e-commerce you may identify a trend: finding new ways to acquire customers, for example SMS as a new marketing channel.
You can develop some of these themes and ideas by spending time coming up with business ideas and themes that interest you. Thinking about these business ideas conceptually can give you the opportunity to think through different aspects of these models. Looking at different "Request for Startups" lists can be the starting point.
Evaluate the investment themes of other firms and investors. What are they paying attention to? How do they codify those lines of thinking?
As you build a library of ideas and themes take the time to articulate your ideas somewhere public (blog, twitter, etc.) Build a "fantasy portfolio" of companies that you really believe in. Even if you don't have money to invest, use it as a way to plant your feet somewhere and have an opinion.
https://twitter.com/hnshah/status/1344730133026914305?s=21
https://twitter.com/zachperret/status/1344351286662221824?s=20
An Aspiring VC's Guide to Building an Investment Thesis
Investment Thesis Collection from Venture Capital Firms -
The Ultimate Guide for Becoming an Idea Machine - James Altucher