Below, you can find various resources for BPMN:

• Official standard documentation: http://www.omg.org/spec/BPMN/
• Tutorial: https://camunda.com/bpmn/
• Simplified unofficial reference: https://camunda.com/bpmn/reference/
• BPMN Poster: http://www.bpmb.de/images/BPMN2_0_Poster_EN.pd

Video with an introduction to BPMN:

Video about gateways in BPMN:

This video details use of business objects:

This video details use of resources in BPMN: