Start at the Dynamics 365 for Customer Engagement Developer Center
The CRM Developer Center https://msdn.microsoft.com/en-us/dynamics/crm/crmdevelopercenter.aspx
Download the SDK https://www.microsoft.com/en-us/download/details.aspx?id=50032
You also have access to a full suite of training materials by logging into Customer Source https://mbs.microsoft.com/customersource/northamerica
Or if you are a partner or Microsoft employee
The Partner Learning Portal https://mbspartner.microsoft.com/Landing
You can also reference blogs and link lists within the world of blogs to find even more resources and training such as the Virtual XRM User Group
Lastly there are a set of YOUTUBE VIDEOS that can help, and the Microsoft Dynamics YouTube Channel
like these
XRM in a Nutshell I: Defining the Data-centric Business Application built using Dynamics CRM https://www.youtube.com/watch?v=XbmYhAy3ouM
and this one https://www.youtube.com/watch?v=G_WPaHZZk7k
And this one https://www.youtube.com/watch?v=L7MRO7ttRIM