Hi Jakob,
As an starting point you can start developing persistence layer for the application. You can leverage your expertise of java background for that. You may start with following points -
- EJB services
- Web Services
- Rest services
- CAF services
Once you are comfortable of using java technologies in sap you can proceed with the front end in below mentioned order
- Web DynPro Java
- SAP UI5
Once you are done with above points you can embark on sap ecosystem based on your interest and skills.
Good Luck.