How to associate a dropdown control with a data source in ASP.NET

I am new to C #.

I have a project for creating an HR system, and I created a page for adding employees, but the supervisor asked me to create a drop-down list that displays the department when adding a new employee.

I do not know how to start and what I should do first. I already added a drop-down list from the tools, but I don’t know how to select the data source and the name and value of this. Should I choose a department table or employee table?

public partial class _Default : Page
{

    private String strcon = ConfigurationManager.ConnectionStrings["hr"].ConnectionString;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)

            bindgrideview();
    }
    protected void bindgrideview()
    {
        SqlConnection strcon1 = new SqlConnection(strcon);
        strcon1.Open();
        string ADDStr = "SELECT F_name , L_name , salary FROM Employee ";
        SqlCommand ADDCmd = new SqlCommand(ADDStr, strcon1);
        DataTable table = new DataTable();


        SqlDataAdapter adapter = new SqlDataAdapter(ADDCmd);

        adapter.Fill(table);
        GridView1.DataSource = table;
        GridView1.DataBind();

    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        string F_name = TextBox1.Text;
        string L_name = TextBox2.Text;
        int status = 1;
        string salarystr = TextBox3.Text.ToString();
        int salary = Int32.Parse(salarystr);
        SqlConnection strcon1 = new SqlConnection(strcon);
        strcon1.Open();
        string ADDStr = "ADDEMP";
        SqlCommand ADDCmd = new SqlCommand(ADDStr, strcon1);
        ADDCmd.CommandType = CommandType.StoredProcedure;
        ADDCmd.Parameters.AddWithValue("@F_name", F_name);
        ADDCmd.Parameters.AddWithValue("@L_name", L_name);
        ADDCmd.Parameters.AddWithValue("@status", status);
        ADDCmd.Parameters.AddWithValue("@salary", salary);

        ADDCmd.ExecuteNonQuery();
        bindgrideview();
        TextBox1.Text = "";
        TextBox2.Text = "";
    }

And this is a screenshot of my page: http://store2.up-00.com/Nov12/YXb11858.png

this is the last code in which there is no error, but there is no element in the drop-down list :(

public partial class _Default : Page
{

    private String strcon = ConfigurationManager.ConnectionStrings["hr"].ConnectionString;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)

            bindgrideview();
    }
    protected void bindgrideview()
    {
        SqlConnection strcon1 = new SqlConnection(strcon);
        strcon1.Open();
        string ADDStr = "SELECT F_name , L_name , salary FROM Employee ";
        SqlCommand ADDCmd = new SqlCommand(ADDStr, strcon1);
        DataTable table = new DataTable();


        SqlDataAdapter adapter = new SqlDataAdapter(ADDCmd);

        adapter.Fill(table);
        GridView1.DataSource = table;
        GridView1.DataBind();

    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        string F_name = TextBox1.Text;
        string L_name = TextBox2.Text;
        int status = 1;
        string salarystr = TextBox3.Text.ToString();
        int salary = Int32.Parse(salarystr);
        SqlConnection strcon1 = new SqlConnection(strcon);
        strcon1.Open();
        string ADDStr = "ADDEMP";
        SqlCommand ADDCmd = new SqlCommand(ADDStr, strcon1);
        ADDCmd.CommandType = CommandType.StoredProcedure;
        ADDCmd.Parameters.AddWithValue("@F_name", F_name);
        ADDCmd.Parameters.AddWithValue("@L_name", L_name);
        ADDCmd.Parameters.AddWithValue("@status", status);
        ADDCmd.Parameters.AddWithValue("@salary", salary);

        ADDCmd.ExecuteNonQuery();
        bindgrideview();
        TextBox1.Text = "";
        TextBox2.Text = "";
        TextBox3.Text = "";
    }
    protected void bindDepartments()
    {
        SqlConnection strcon1 = new SqlConnection(strcon);
        strcon1.Open();
        string ADDStr = "SELECT ID,department_name FROM Department ";
        SqlCommand ADDCmd = new SqlCommand(ADDStr, strcon1);
        DataTable table = new DataTable();


        SqlDataAdapter adapter = new SqlDataAdapter(ADDCmd);

        adapter.Fill(table);

        DropDownList1.DataSource = table;
        DropDownList1.DataValueField = "ID";
        DropDownList1.DataTextField = "department_name";
        DropDownList1.DataBind();

    }

}