Setup –> DataManagement –> Sandbox

The below link is used to describe ‘How to Create or Refresh Sandbox’:

https://ap1.salesforce.com/help/doc/en/data_sandbox_create.htm

For Trial version, just create your login credentials using the below link

https://www.salesforce.com/form/signup/freetrial-force-form.jsp?d=70130000000G3R3