ESRI has tutorials, videos and courses to get you working effectively with ArcGIS Online. They are available from here.

Including:-

• Create maps in Map Viewer
• Share and collaborate using ArcGIS Online
• Perform analysis in Map Viewer
• Learn essential demographic data skills
• Common skills for working with data in ArcGIS Online
• Essential ArcGIS Online skills
• Modern GIS - Introduction to GIS
• BIM and GIS
• Introduction to policy mapping
• Try ArcGIS for emergency management

Documentation to find answers and complete projects:-

• Get started
• Create content
• Analyse data
• Share and collaborate
• Manage data
• FAQ

Videos

• Get Started with ArcGIS Online
• ArcGIS Online: Mapping Basics
• ArcGIS Online: Content Basics
• ArcGIS Online: Analysis Basics

ESRI's YouTube channel is available from here!