asp problem

hi
could you give me help in my problem i am facing problem in asp.net

i want to select a text in the drop list and i would like to keep the text area unwritble until the user select the purpose text in the list

for example we have a drop list wich includes :
name
class
other

and i have a text area the user shoulnt be able to write in theis area until he select only the other text


please could you help me in that