Hi Nahe!
If you want to start with Mendix, look in to the learning paths of Mendix @ https://academy.mendix.com especially the Rapid developer course for a good start.
If you are completely new to Low code development, ensure you get the right guidance after these learning paths. As these are sharing a ton of knowledge, its still like lego. You need to make the right decisions to use all blocks in the best way. An option coul be to join one of our bootcamps to get a deep dive in to learning how to use the platform in the best possible way.
Regarding learning how to integrate Mendix with SAP there are a few learning paths.
But be aware, that you first need to understand how to use the platform in general. Otherwise you will make the integration work and an useless app.
https://academy.mendix.com/link/paths/32/Learn-to-Build-an-SAP-App
https://academy.mendix.com/link/paths/69/Develop-a-Smart-SAP-Catalog
https://academy.mendix.com/link/paths/138/Create-an-SAP-Material-App-with-BAPI-Connector