Hello Subash,
You can integrate Ariba either by PI or Using a Add-On. If you want to use add-on you need to be in ECC6 Enhancement pack 4 or higher and uses SOA.
The link below has the Admin guide for both options.
If you need building blocks for the same I can fish the document out in the following link.
http://service.sap.com/~sapidb/01100035870000093782013E
Buyer - the Ariba side where PO are raised and covers all the aspect of procure to pay.
Hope this helps.
Vijay