Writing use cases

Please refer to Basic Use Case Template

My short note is：

(1) Define actors
E.g. "Customer", "Shipping System"

(2) Write down names of scenarios (i.e. use cases) which may include a number of activities
E.g. "Create Account" , "Log In" , "Check Out"

(3) Group the use cases as system boundary which defines the scope of what a system will be.

(4) Use arrows showing which actors are involved in which use cases

Then you can finish a high-level use diagram.

More Explanation: http://www.developer.com/design/article.php/2109801
Example: http://www.agilemodeling.com/images/models/useCaseDiagram.jpg