It depends on how you do it.
If you are using a BindingSource to load the combobox, then set the DisplayMember to the appropriate field for display in the combobox, and the ValueMember to the field you want in the textbox.
Handle the ComboBox SelectedIndexdChanged event, and set the text:
private void MyComboBox_SelectedIndexChanged(object sender, EventArgs e)
MyTextBox.Text = MyComboBox.SelectedValue.ToString();
(You may want to put some error checking in there, but that gives you the idea)