Microsoft have released a free eBook to help you get started with Microsoft Dynamics CRM 2015. The guide is useful for both Online and On-premise systems.

The eBook goes through an overview of Microsoft CRM and topics such as:
• Importing data
• Navigation
• Searching for data
• Dashboards
• Activity Management
• Sales
• Customer Service

You can Download the Guide here

Edit: Here is a list of useful guides to help you get started with Microsoft CRM Online which we have collated.