Experience. Whatever area you want to work in as a consultant, you must be able to show clients that you’re qualified to advise them. If there are organizations related to your field, make sure that you’re a member of them. Similarly, if you can take a course and become certified or registered in your field, do it and then add that to your credentials.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.