place your cursor in the first text box, then in the tag selector, select the form tag.
go to Window -> Behaviors to open the behaviors panel.
you will need to add Conditional Validation using the select list as the trigger and the text box as the target, then add required validation for the text box.
After these validations are added. use the up arrow key to move them up the list of validations to be just after the required validation for the select list.