How to refer PageReference method from apex test class

I am new to apex, I have created a button to call the apex class through the visual force page. Here is my visual force page code.

<apex:page standardController="Opportunity" 
extensions="myclass" 
action="{!autoRun}"> 
</apex:page>

Here is my apex class.

public class myclass {
    private final Opportunity o;
    String tmp;

   public myclass(ApexPages.StandardController stdController) {
        this.o = (Opportunity)stdController.getRecord();
    }
    public PageReference autoRun() { 
        String theId = ApexPages.currentPage().getParameters().get('id');

   for (Opportunity o:[select id, name, AccountId,  from Opportunity where id =:theId]) {

                 //Create the Order
                    Order odr = new Order(  
                    OpportunityId=o.id
                    ,AccountId = o.AccountId
                    ,Name = o.Name
                    ,EffectiveDate=Date.today()
                    ,Status='Draft'
                    );
                insert odr;
                tmp=odr.id;             
              }                  
        PageReference pageRef = new PageReference('/' + tmp);  
        pageRef.setRedirect(true);
        return pageRef;
      }
}

I want to create test class. I don't know how to refer PageReference autoRun() method from test class. Guys need help if some one can tell me about test class of this apex class.