Click on the gear icon in the upper right hand corner. 

Click on the "New Sandbox" button, in the center of the screen. 

Identify if the source org for the sandbox. It can be your Production org, or another sandbox. 

Click the next button, located under the type of sandbox you wish to create. 

Click create if you do not have a reference to an Apex Class. 

Click on your sandbox after the creation and activation has complete.  This is dependent on the size of your source org for the sandbox. 

Log into your sandbox by clicking on the login button. 

Note the user name with the .[sandbox name] appended. 

Enter your Salesforce password.  And complete the login.  Take a tour of your sandbox.  

Salesforce Sandbox Creation