It would be fun to tackle this a few ways. One you could build the API calls first and just use some thing like postman to really understand how APIs work and then build the front end around it, or you could start just thinking directly about the flow of the actual app itself. So if it’s a chat bot, you’re going to need a text input field you’re going to need a submit button, how do you write a text input field? How do you write the code for a submit button? If you’re using something like react or vue a lot of that is out of the box, but you still have to learn how to do it. Then you would need some type of backend that’s going to make the calls to the external API , and then you’re going to need to learn how to receive the responses from the API and render them. In the end, if you can build a basic chat bot, you will have a solid foundation of whatever library you’re using, and APIs.  Then you can just expand and build a more complex apps from there.

there will be some foundational concepts that you won't learn through this process that you may need to go back and learn if you want maybe a career as a developer but if your goal right now is to just build an app I would say just start having fun.