When I asked a question, I've got the answer like this:
Please follow the below link. It will be helpful for you.
1.) http://www.salesforcetutorial.com/create-salesforce-application/
2.) https://help.salesforce.com/articleView?id=dev_tabsetcreate.htm&type=0
3.) https://ccoenraets.github.io/salesforce1-app-tutorial/create-expenses-tab.html

What is the difference and relationship between these document sites?
http://www.salesforcetutorial.com
https://help.salesforce.com
https://ccoenraets.github.io

Should I read them all? Are there any repeated content, that will take a lot of my time to read?

Thanks!

When I log in to salsforce by my developer account,and study follow trailhead, I can complete Hands-on Challenge in my own personal Salesforce environment,is this personal Salesforce environment my org? If not, what is my org in salesforce?

What is the different between my org, sandbox and production envirnoment?

Is sandbox public for everyone? Or I have to apply for my sandbox?

When I log in to salsforce by my developer account,and study follow trailhead, I can complete Hands-on Challenge in my own personal Salesforce environment,is this personal Salesforce environment my org? If not, what is my org in salesforce?

What is the different between my org, sandbox and production envirnoment?

Is sandbox public for everyone? Or I have to apply for my sandbox?