Create a microservice for other apps

Question

I want to create a software that any app could integrate into itself, to provide a microservice to that existing mobile application. An example it could be a method of payment with its interface or whatever. The thing here is that i don't even know where or what do I have to start looking and learning because I'm a little begginer to programming. So it would be great if anyone can give me his opinion on the topic and recommend some concepts or technologies or programming languages it would be really helpful. Thanks!