You need to first become a domain expert in some area. Then try to look for problems and areas of improvement in that domain. Notice what kinds of problem users are having and what solutions may already exist. Get involved in the ecosystem surrounding that domain. This will be a source of inspiration and ideas for a solution that you can build. The next step is to build a prototype as fast as possible and get it in front of users you think will get value out of it and see if they will actually pay for the prototype. Keep iterating and try to grow business from there.