Sounds simple enough: here are a few articles to read. Just include the parameters in your other dataset code.

http://technet.microsoft.com/en-US/library/aa337498(v=SQL.105).aspx

http://msdn.microsoft.com/en-us/library/dd255197.aspx

http://www.resultdata.com/adding-cascading-parameters-to-a-ssrs-report/

Google search on SSRS Cascading Parameters for more articles.