I have no experience writing API documentation, but a need has arisen for it at my company. However, the only guides I can find for API Documentation are for APIs that use JSON, XML, Java, etc. What is the best way to dive into API documentation using a C# document as a first project?
Look at API documentation for C#, such as that on the Microsoft site (see here), or check out one of the multitude of repositories on Github.
Basically, API document the ways users interact with your product. You have to specify fields, classes, methods, constants, etc. Look at the Java API to see a systematic way to do this and then adapt it to your product.
If the API you're going to document is a REST API, Tom Johnson has this excellent free online course, Documenting REST APIs.
If that doesn't fit your needs, Stack Overflow has a couple of topics that might also be interesting: