Increasing Textbox amount as button is clicked.

Question

 Hello,

 I have to make a soda machine software. I'm very new to programming and I'm trying to make this thing work. I have buttons for Dollar note, quarters, dime, nickel. And what i want is the user to click on it and show the incremented amount in textbox. I'm just so frustrated because I simple don't know how to make it work. i tried ++dollarAmount ad everything but no success. 

Im putting the code to form1.cs and I've also attached the entire zip file if you want to take a look at it.

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

namespace VendingMachine

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

            txtboxInstructions.Text = "Please click on the amount that you would like to enter.";

        }

        private void btnClear_Click(object sender, EventArgs e)

        {

            txtBoxAmount.Text = "";

            txtBoxSelection.Text = "";

        }

     }

}