Could you please clarify what you really mean by the app development as an "app" is one of the most versatile words of computer science. There are multiple categories in app development from mobile, to web and to cloud.
So, here i will try to give you some basic concept for app development in most of the scenario abd then you could have a start for yourself. So, here's how you could make an app:
1. Desktop: Desktop apps are commonly known as softwares and are created using the help of programming language and a compiler. They are usually the easiest among all types of apps to create.
2. Android: To create an app in Android, you have to download Android Studio, an open source software from Google and Adb. Then you could make native android app with the help of coding in xml and java/c++.
3. iOS: The apple doesn't provide you with open source material to develop their app. But instead but they have an max exclusive compiler called Xcode. The xcode works in swift language to create ios apps.
There are other types of apps too like web app and cloud apps, but to understand those we would have gotten too deep. That's all for now...