Design Pattern for consuming API's

What I am trying to achieve is simple, I have many clients (they increase by the time), they all have API's all of them are different, some are xml some are json and all in different format but they all have the same purpose.

so I am building a middleware to make all these apis as standard and return a standard JSON for all of them so I can use them in my website and mobile app. I have a good knowledge in php and javascript but I want to create the project using go language.

I know the design pattern doesn't affect that, the problem is, I am so confused to know how to start, if anyone can help me with a blueprint or how I can start the structure of my project I really appreciate it.

also I want to add, I don't want to store everything on my database just logging, the data should be on demand.

This image is example of what I think I need to do.