In the first place, decide on the platform. If you want to develop both iOS and Android versions at the same time, you should hire two developers. Write a job description with specific tasks and requirements. Post your job on the specialized Telegram channels for IT professionals and on job boards. In this way, you can find a developer of the junior or, at best, middle level. It's more efficient to search for competent middle and senior developers via LinkedIn. And it's even better to contact a recruitment agency that can find the best specialist for your project.