Well, first you need to find a reliable manga app. For example, Tachiyomi is a good choice. Then, open the app and search for the manga you want to read. Make sure you have a stable internet connection.
To read manga on iPhone, you could try using Kindle app and purchase manga ebooks. Also, check out Webtoon, it has a good collection. Just make sure your iPhone has enough storage and updated software.
You can download manga reading apps from the App Store. Some popular ones are Manga Storm and Manga Reader. Just search for them and install.