What skills do I need to possess to create an application?

Answer 1

You need to know a programming language that you can use to create an application

Answer 2

It really depends on what kind of app you want to create on which platform.
For a simple calculator app on Windows C# is just fine, on Android Java is enough.
If you want your application to store data online, you need to have a server or create one for free on 000webhost or create one with XAMP. You also have to setup a database which requires SQL knowledge.