I am new to JMeter and I want to learn JMeter step by step. Would you suggest me the easiest ways that how can I start a performance testing using JMeter??
I believe the best option to start with is JMeter official User Manual.Given you read it end-to-end and execute mentioned examples to get understanding on how this or that test element works - you'll be good to go to start load testing using JMeter.
More detailed information on each Test Element can be found at Component Reference page.
Information regarding JMeter Functions is at Functions and Variables page
I would also suggest bookmarking The Ultimate JMeter Resource List - the most comprehensive compilation of JMeter-related materials.