Hi Chandrajith,
As suggested by Selva, you have to take a basic SAP ABAP training to get into SAP. SAP ABAP Webdynpro exactly suits you because "The concept of Web Dynpro ABAP is identical with Web Dynpro Java and offers more or less the same functions" - SAP, so you can utilize the Java skill very well.
Also, suggested by others, you can do a google search on SAP ABAP Webdynpro.
There is separate SAP SCN community forum for SAP ABAP Webdynpro Web Dynpro ABAP and also check following SAP links as a starting point https://help.sap.com/saphelp_nw70/helpdata/en/77/3545415ea6f523e10000000a155106/frameset.htm & Web Dynpro for ABAP: Tutorials for Beginners
Regards
RC Gopi