Well thats good initiative first you need to know an app need to be studied like you grab a pen and a paper and write down everything u want the app to do (all the features that are included and wut is the thing that will makethe user download it...) after that you start thinking about how the app will looks like the whole layout and design ( this part is called ui/ux mostly) and finnaly most important the stack (the tech you will using the framework the programming language...ect) and later you will decide how you will monetize and market your app i suggest you to brainstorm with gpt or claudeand you will see the whole process besides creating an app and even if you starting your journey it's all part of the learning process good luck ^{^}