Hi Krishna,
If you want to learn how sales assistant works you have to focus on Sales Methodology. Sales assistant is included into Sales Metholodogy functionality.
Please check the next links:
How to create Sales Assistant for Opportunity Management..
SAP CRM 7.0 - Sales Methodology - YouTube
http://www.saptraininghub.com/sap-crm-sales-cycle/
Sales Assistant - Opportunity Management - SAP Library
I hope that helps.
Best regards,
Jorge Ocampos.