You need to make some assumptions about who your ideal client is, what their interests are, what their pain points are and what you think they need help and support with. Once you have done that, you can then find where people that fit those categories and criteria are spending time and work at proving or disproving those assumptions to build a clear picture of who your ideal customer is and what they are all about.
Online there are lots of different outlets that you can utilize, from joining Facebook groups that are specific to your niche, to following similar social media accounts and interacting with their audience, asking questions in relevant forums, making new connections through online networking events. All the while you can use these outlets to test the assumptions that you've made about your audience.
If you already have an established audience then you can talk to them, you can survey them you can ask them questions about what their struggles are and their pain points are, what inspires and motivates them, and you can develop your offering around the answers that you get from those questions.
Once you are clear on what your audience enjoys, dislikes, struggles with and is motivated by, you can build your customer persona.
If you'd like some support to build out your customer persona, download our free workshop on Creating Your Own Living Business. You'll find a template within the workshop to help you define and create your customer persona.