Hi Ejas,
From my own experience:
1. SAP ABAP developing: mainly ABAP proxies. Learn about IDOC and RFCs. Pay attention to know main BAPIs or IDOCs about the ECC modules installed in your client.
2. Learn connectivity without PI, SAP Netweaver Gateway or SAP webservice run-time.
3. SAP Netweaver Developer Studio.
4. SAP TCP-Gateway tool to debug http and mail scenarios: Troubleshooting SOAP, HTTP and Mail Adapter Scenarios with TCPGateway
5. Collect PI debug traces, XPI Inspector: Michal's PI tips: XPI inspector - help OSS and yourself
Regards.