I am creating billing app not sure where I am going wrong

Question

Bill.cs

using System;
using System.Collections.Generic;
using System.Text;
namespace ConsoleApp2
{
public class Bill
{
public int BillNumber { get; set; }
public DateTime BillDate { get; set; }
public List LineItems { get; set; } = new List();
public void AddBillLine(BillLine billLine)
{
LineItems.Add(billLine);
}
public void RemoveBillLine(int SOMEID)
{
throw new NotImplementedException();
}
///
/// GetTotal should return the sum of (Cost * Quantity) for each line item
///
public decimal GetTotal()
{
throw new NotImplementedException();
}
///
/// MergeBill appends the items from the sourceBill to the current bill
///
/// Bill to merge from
public void MergeBill(Bill sourceBill)
{
throw new NotImplementedException();
}
///
/// Creates a deep clone of the current bill (all fields and properties)
///
public Bill Clone()
{
throw new NotImplementedException();
}
///
/// Outputs string containing the following (replace [] with actual values):
/// Bill Number: [BillNumber], BillDate: [dd/MM/yyyy], LineItemCount: [Number of items in LineItems]
///
public override string ToString()
{
throw new NotImplementedException();
}
}
}
BillLine.cs
namespace ConsoleApp2
{
public class BillLine
{
public int BillLineId { get; set; }
public string Description { get; set; }
public int Quantity { get; set; }
public double Cost { get; set; }
}
}
 Program.cs

using System;
using System.Collections.Generic;
namespace ConsoleApp2
{
public class Program
{
static void Main(string[] args)
{
Console.WriteLine("Billing app started....");
CreateBillWithOneItem();
CreateBillWithMultipleItemsAndQuantities();
RemoveItem();
MergeBill();
CloneBill();
BillToString();
}
private static void CreateBillWithOneItem()
{
var bill = new Bill();
bill.AddBillLine(new BillLine()
{
BillLineId = 1,
Cost = 6.99,
Quantity = 1,
Description = "Apple"
});
Console.WriteLine(bill.GetTotal());
}
private static void CreateBillWithMultipleItemsAndQuantities()
{
var bill = new Bill();
bill.AddBillLine(new BillLine()
{
BillLineId = 1,
Cost = 10.21,
Quantity = 4,
Description = "Banana"
});
bill.AddBillLine(new BillLine()
{
BillLineId = 2,
Cost = 5.21,
Quantity = 1,
Description = "Orange"
});
bill.AddBillLine(new BillLine()
{
BillLineId = 3,
Cost = 5.21,
Quantity = 5,
Description = "Pineapple"
});
Console.WriteLine(bill.GetTotal());
}
private static void RemoveItem()
{
var bill = new Bill();
bill.AddBillLine(new BillLine()
{
BillLineId = 1,
Cost = 5.21,
Quantity = 1,
Description = "Orange"
});
bill.AddBillLine(new BillLine()
{
BillLineId = 2,
Cost = 10.99,
Quantity = 4,
Description = "Banana"
});
bill.RemoveBillLine(1);
Console.WriteLine(bill.GetTotal());
}
private static void MergeBill()
{
var bill1 = new Bill();
bill1.AddBillLine(new BillLine()
{
BillLineId = 1,
Cost = 10.33,
Quantity = 4,
Description = "Banana"
});
var bill2 = new Bill();
bill2.AddBillLine(new BillLine()
{
BillLineId = 2,
Cost = 5.22,
Quantity = 1,
Description = "Orange"
});
bill2.AddBillLine(new BillLine()
{
BillLineId = 3,
Cost = 6.27,
Quantity = 3,
Description = "Blueberries"
});
bill1.MergeBill(bill2);
Console.WriteLine(bill1.GetTotal());
}
private static void CloneBill()
{
var bill = new Bill();
bill.AddBillLine(new BillLine()
{
BillLineId = 1,
Cost = 6.99,
Quantity = 1,
Description = "Apple"
});
bill.AddBillLine(new BillLine()
{
BillLineId = 2,
Cost = 6.27,
Quantity = 3,
Description = "Blueberries"
});
var clonedBill = bill.Clone();
Console.WriteLine(clonedBill.GetTotal());
}
private static void BillToString()
{
var bill = new Bill()
{
BillDate = DateTime.Now,
BillNumber = 1000,
LineItems = new List()
{
new BillLine()
{
BillLineId = 1,
Cost = 6.99,
Quantity = 1,
Description = "Apple"
}
}
};
Console.WriteLine(bill.ToString());
}
}
}

Can anyone please help on this I want to know how I can create bill for one item,  create bill with multiple items and quantities, remove item, merge bill, clone bill.