Get your apaleo developer account and check out this guide to learn how to connect to the apaleo APIs.
You can find the swagger documentation on https://distribution.apaleo.com/swagger/index.html.
All apaleo APIs are described as Swagger documents. That lets you generate API clients directly from the swagger.json files. There is a large community providing client generators for almost every language. For example, swagger code generator is a good project, which allows the generation of API clients for Java, PHP, C#, NodeJS and more. This quickly gets you up to speed, and as a bonus, you can access API documentation within the auto-generated methods and models.