OAuth and JSON Communication to second server

Hello there,
today I have a, maybe, really complex task for you :smiley:

I am building a website for a dancing school which works together with an administration company.
The school’s website should show the offered courses, For this one need to establish a connection via OAuth 2.0 . It works like GitHub API v3 .
Than the customer can see the courses like here at an other dance school https://www.ring3.de/erwachsene/gesellschaftstanz/stufe-1.html when you click on “Termine”.

Selecting a course the customer has to fill out a form. The submission goes as a JSON back to the authenticated server of the administration company. You can see the PHP site here. Do not worry, you can fill the form out until the part “Allgemeine Geschäftsbedinungen” where you have to check the policy.

At the moment all courses are displayed as an iFrame.
Also the site is not at its final form.

I would appreciate some help from you great designers and programmers out there.
I learned media design print in Germany and have only a limited knowledge about coding and websites. A lot of it I learned by myself. Why I am doing such a huge job? The owner of the dancing school and my client, is my step-dad, who wants to stop to work with the former internet agency.’



P.S. When you are able to read and understand german, I can see the official API documentation and the API extension for dancing school