how to do this using combo box

You can write your code on ComboBox OnSelectedIndexChanged Event to fill your TextBox with appropriate value. You can write your own reusable function for this. See the below code :

''' <summary>
''' Fille TextBox Based On Value Selected in ComboBox.
''' </summary>
''' <param name="SelectField">Table Column Name To Display In TextBox As String</param>
''' <param name="TableName">Table Name As String</param>
''' <param name="ColumnName">Table Column Name For Where Clause</param>
''' <param name="ComboBoxName">ComboBox Name For Where Clause Condition</param>
''' <param name="SetTextBoxName">TextBox Name To Fill Value</param>
''' <remarks></remarks>
Public Sub FillTextBox(ByVal SelectField As String, ByVal TableName As String, ByVal ColumnName As String, ByVal ComboBoxName As ComboBox, ByVal SetTextBoxName As TextBox)
	Dim _ObjDataAdapter As SqlDataAdapter = New SqlDataAdapter
        _ObjDataAdapter.SelectCommand = New SqlCommand
        _ObjDataAdapter.SelectCommand.Connection = _Connection 'Your SqlConnection Object
        _ObjDataAdapter.SelectCommand.CommandText = "Select " & SelectField & " FROM " & TableName & " WHERE " & ColumnName & "='" & ComboBoxName.Text & "'"
        Try
            _ObjDataReader = _ObjDataAdapter.SelectCommand.ExecuteReader() 'Exectuing query
        Catch ex As System.InvalidOperationException
            MessageBox.Show(ex.ToString)               'Providing Error Message
        End Try
        While _ObjDataReader.Read                      'Reading DataReader Items
            SetTextBoxName.Text = _ObjDataReader.Item(0)'Adding these Items Into ComboBox
        End While
        _ObjDataReader.Close()                             'Closing DataReader Object
        _ObjDataAdapter.SelectCommand = Nothing
End Sub

After that you can call this Function on ComboBox OnSelectedIndexChanged Event

FillTextBox("DisplayColumn", "TableName", "WhereClauseColumn", ComboBox1, TextBox1)

I hope this will help you. :)