I need to disable a drop down when i check a checkbox

I am pretty new to drupal......

I am trying to disable a drop down when i check a checkbox. I know that javascript has to be used and have looked into many posts... Still I haven't found the right suggestions. What is the drupal syntax in javascript to achieve that. Mostly they give jquery suggestion with onchange events but i need a simple javascript that checks whether the particular checkbox has been checked and disable the drop down. Its urgent please and any help would be greatly appreciated