Start Learning Silverlight using following resources:http://www.silverlight.net/getstarted/
http://channel9.msdn.com/Learn/Courses/Silverlight4
Silverlight ComboBox Sample for RIA Services
http://blogs.msdn.com/b/kylemc/archive/2010/06/18/combobox-sample-for-ria-services.aspx

  WCF RIA Services
http://msdn.microsoft.com/en-us/library/ee707344(v=VS.91).aspx

http://www.silverlight.net/learn/videos/silverlight-videos/

Silverlight Video Tutorials:

Blogs:
http://wildermuth.com/