如何在C＃中创建APi？

I have been doing programming, but now I'm facing a challenge. I'm on a project where I have to create an API in C# for my users. The basic process is that I have a C# code where I'm reading an XML file and obtaining the attributes and values that are between the tags. I achieved that and now I have to create consumable API for the same functionality. Is there any way through which I can learn to create API?I have been doing programming, but now I'm faci