That is a legitimate question, but a bit vague.
Is your question about learning how to develop ?
If so, learn from the many Apple books (free) and start with "Intro to App development with Swift".
Or is your question about finding a good app idea ?
That is the important question. There is no magic recipe here, but I can give an advice for one method to find it: think of problem you face in your daily life, that is really a pain point, and see how an app could help.
Or something else ?
In anycase, wish you good luck. And remember you will always find helping hands in the forum for your technical questions.