Pentaho reporting cannot be used for creating dashboards - however you can embed pentaho reports within a dashboard if you want to.
You need to look at the "Ctools" from webdetails. Start with the ctools-installer script then take a look at the CDE product. This is an very complete dashboarding framework. There is documentation on the ctools websites, and if you get really stuck you can buy some very cheap tutorials which are pretty good. ( There are tutorials available from webdetails or ambientbi - take your pick! Both are based on CDE )
Webdetails are the main developer of CDF which is the dashboard framework that Pentaho themselves use for their EE products - so its a good product!