SE38 -> Utilities -> Settings -> ABAP Editor -> Debugging
Activate the external debugging and choose the New Debugger option in ABAP debugger.
Go to the particular place in the code and put break point, pop will appear then choose the HTTP break point.
If you are triggering the RFC from SAP portal make sure that both the user ID should be same
If the users are different then provide the XI/Portal User ID in the users field.