You will build a questionnaire (survey) and conduct in-person and online interviews.
Then you will need to analyse the responses to inform the build of your product. Then and only then you can start developing your solution as you will know who you are building it for and that they are willing to use it.
Using online tools help you create surveys that you can post on your social media or by email and get people to fill in. This will complement greatly the in-person interviews that you will conduct, with the same survey of course so that you obtain consistent information to work from.
The following online tools help you collect the information in a easy to access spreadsheet, from which you can analyse the information you get. Once you collected plenty of information from your potential customers be sure to spend some time reviewing it to use it effectively for building your solution.