Hi Ram
As Suman suggested, indeed SAP tutorial is one of the best way to get started with dashboard or any SAP application tool.
I will also recommend reading the user guides in accordance to the version of dashboard which is currently being used in development.
SAP BusinessObjects Dashboards 4.1 – SAP Help Portal Page
All Products – SAP Help Portal Page
This will give you complete information of all SAP BO products with version wise document.
BO Dashboard cookbook referred by Suman is a great way to tweak and use the best practice method to develop a compact and flexible dashboard,
I have shared some of the tutorial materials to mentioned email id.
---
Regards
Kishore Balachandran