Office 365 - Getting Started!

Below are a few articles that will help you get started with developing solution for Office 365

Exchange Online Overview
https://msdn.microsoft.com/en-us/Office365TrainingCourse_1V_3

Developing Messaging Solutions for Exchange Online
https://msdn.microsoft.com/en-us/office365trainingcourse_session-10_unit

Exchange Online Technical Articles
https://msdn.microsoft.com/en-us/library/gg193994(EXCHG.140).aspx

Office 365 Developer Training Course
https://msdn.microsoft.com/en-us/Office365TrainingCourse

Enjoy!