Below are a few articles that will help you get started with developing solution for Office 365Exchange Online Overviewhttp://msdn.microsoft.com/en-us/Office365TrainingCourse_1V_3
Developing Messaging Solutions for Exchange Onlinehttp://msdn.microsoft.com/en-us/office365trainingcourse_session-10_unit
Exchange Online Technical Articleshttp://msdn.microsoft.com/en-us/library/gg193994(EXCHG.140).aspx
Office 365 Developer Training Coursehttp://msdn.microsoft.com/en-us/Office365TrainingCourse
Enjoy!