Your best place to start is online. The local Apple store will have intro-level, 1 hour classes but that is not nearly enough for the level of proficiency required to publish apps. After setting up your environment, your best bet is to search for tutorials online such as on raywenderlich.com.
The only way to learn this skill, is by doing. That is, you must write several practice apps.
Realistically, it will take you several months of practice to begin to understand mobile application development – including aspects such as battery life, memory usage, and flaky internet connection, which specifically affect mobile devices. This timeline applies even if you are already proficient in other development stacks, and will take longer if you are brand new to development.
Be aware that typically mobile application development does not occur in isolation. It is very common for a mobile application to communicate with a remote server - referred to as a “backend” - over the internet and exchange data over an application programming interface (API). When you start building robust mobile apps, you either have to work with an existing backend, or learn to build your own. Building your own server will require additional skills beyond mobile app development.