Download csv File using Visual force page

I tried downloading a csv file from a visualforce page.

In the code below i am querying contacts object and downloading the query result in csv

 {!a.FirstName},{!a.lastName}

Apex Class:

public class Sample
{   
    public List contacts {get;set;}
    public pageReference fetch()
    {
         contacts = [SELECT Id,firstname,lastname FROM Contact limit 20];
        return null;
    }    
}