Need help with Apex test code

Question

So, Ive managed to learn a bit of Apex code. Now I need help with associated test code. Could someone please help me get started?

public class multiAddCEx {

   public Id cID = ApexPages.currentPage().getParameters().get('Id'); //grab the Vendor Bill ID
   //Integer count = 0;
   string num_name = '';
   // Lookup against Available Cost Codes Junction Object
   public List<SelectOption> costcodes {get; set;}
   public multiAddCEx (){
      costcodes  = new List<SelectOption>() ;
      for(Cost_Code_Connection__c cc : [select Cost_Code_Junction2__r.Cost_Code_ID__c , Cost_Code_Percentage__c, id,name from Cost_Code_Connection__c where Cost_Code_Connection__c.Bill_Item_Junction__c =: cID order by Cost_Code_Junction2__r.Cost_Code_ID__c, Cost_Code_Percentage__c ]){
          costcodes.add(new SelectOption(cc.Cost_Code_Junction2__r.Cost_Code_ID__c +' - '+ cc.Cost_Code_Percentage__c +'%',cc.Cost_Code_Junction2__r.Cost_Code_ID__c +' - '+ cc.Cost_Code_Percentage__c +'%' )) ;
      }
   }
   //Lookup to find User Intranet ID in User records
   public List<SelectOption> intranetids {get; set;}
   {
        intranetids = new List<SelectOption>() ;
        intranetids.add(new SelectOption('-Select-','-Select-'));
        for(User ui : [select Intranet_ID__c , id, name, IsActive from User where IsActive = True order by Intranet_ID__c]){
            if (ui.Intranet_ID__c != null){ 
                num_name = ui.name;
                intranetids.add(new SelectOption(ui.Intranet_ID__c, ui.Intranet_ID__c +' - '+ num_name  )) ;
            }
        }    
    }
    List <Bill_Allotment__c> CExList;
    public Id getID {get; set;}
    public PageReference reset()  { //pull at most 8 expense records to show so we don't clutter up the page
        CExList = [select name, Bill_Allotment__c.Phone_Number__c ,  Allotment_Amount__c, Bill_Allotment__c.Allotment_User_Id__c,  Bill_Allotment__c.Bill_Cost_Code__c  from Bill_Allotment__c where Bill_Item__c =: cID order by createddate limit 0 ];
        return null; 
    }
    public List <Bill_Allotment__c> getCExs() {
    if(CExList == null) reset();
       return CExList;
    }
    public void setAccounts(List <Bill_Allotment__c> cexs) {
         CExList = cexs;
    }
    public PageReference save() {//upsert records on save
       try{
           upsert CExList;
           ApexPages.Message myMsg = new ApexPages.message(ApexPages.Severity.Info, 'Records Saved Successfully'); //show confirmation message on save
           ApexPages.addMessage(myMsg);
           return null;   
       }
       catch(DmlException ex){
           ApexPages.addMessages(ex);
       }
       return null;       
    }
    //ApexPages.Message myMsg = new ApexPages.message(ApexPages.Severity.Info, 'Records Saved Successfully'); //show confirmation message on save
    //ApexPages.addMessage(myMsg);
    //return null;}
    public PageReference cancel() {//close window without upsert
       return null;
    }
    public PageReference add() {
       //count++;
       CExList.add(New Bill_Allotment__c(Bill_Item__c = cID)); //add records to Bill Items and associate with current Bill
       return null; 
    }
    //public Integer getCount() {
       //return count;
    // }
}

Answer 1

You can find documentation about how to write unit tests:

Adding a test class

An introduction to Apex Test methods

Answer 2

Trailhead also have an unit dedicated to Apex testing https://developer.salesforce.com/trailhead/trail/force_com_programmatic_beginner