Hi Can someone please help me write a test so I can deploy an APEX class.

I had to edit some previous code that someone wrote and I am try to learn APEX and Force and read some articles on testing but don't have a clue on how/ what to test in my code.  Coming from a web designer background you just test it in the browser and  it's all good.

public with sharing class EventListsStates {

    private static string prodId;
    private static string noRows;
            
    public EventListsStates() {
        prodId = ApexPages.currentPage().getParameters().get('prodId');
        noRows = ApexPages.currentPage().getParameters().get('noRows');
        noRows = (noRows == null || noRows == '') ? '5' : noRows; 

    }
  
    public Campaign campaign {get ;set ;}

   public list<Campaign> upcommingEventsNSW {get {
        if (upcommingEventsNSW == null) {
            
            upcommingEventsNSW = new List<Campaign>([Select Name,Opportunity_Category__c,Event_Date__c,Venue_Name__c,Event_Month__c,Event_Day__c,Address_Details__c, StartDate, Running_Times__c, Product_Name__c, Location__c, IsActive From Campaign  where Location__c = 'Sydney' and startdate >= today and IsActive = True and Publish_on_the_web__c = true and Product_Name__c = :prodId order by startDate ASC limit :integer.valueOf(noRows)]);
        }
        return upcommingEventsNSW;
    } set;}
    
  
        
}

 Thank you!!

Joe