I'd like to know how to create a dropdown combo and populate it in vb.net/asp.net

Using the Form Designer, simply drag a ComboBox onto your form. This will by default called "ComboBox1"

In the Load event for the page, you could include:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        ComboBox1.Items.Add("One")
        ComboBox1.Items.Add("Two")
        ComboBox1.Items.Add("Three")
        ComboBox1.Items.Add("Four")
        ComboBox1.Items.Add("Five")

    End Sub

This would add the five items to the drop-down box.

You could use a loop to populate the list:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        For f As Integer = 1 To 1000
            ComboBox1.Items.Add(f)
        Next

    End Sub