Here are some great resources you will need to get started creating your own custom reports.
Here is how you can get access to reporting data sources in RTC and Jazz. One option is to use OSLC CM REST interfaces. The other is to use the repository reporting REST interfaces. I used the latter.
Use the examples provided at the bottom of the Wiki to create the right URLs to get report data you need. The URLS provide a self documenting schema. To quickly see the available name value pair options point your browser to for example https://jazz.net/hub/ccm/rpt/repository
You can then select the urls for the items you want to display.. workitems, builds, scm etc.
Where might you get tripped up?
Internal Reportable REST API Wiki – https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#workitem
OSLC Workshop & Tutorials – http://jazz.net/library/article/477
How to consume the RTC Change Management Services – http://jazz.net/library/article/352/
URL Formats for getting the right data from RTC https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#workitem