Hi & welcome:
You will need to apply two different techniques. First, conditional formatting on your text box control -- the condition would be that if the field for the drop down does not equal the value where the text box should be filled out, hide the text box. The second is data validation -- on the field for the text box, add data validation. The condition would be that if the field for the drop down is equal to the value where the text box should be filled out and the text box is blank, then the field is not valid.