1) Go to Admin Menu->Field Layout->Custom Field and create a "Type" field (or whatever you'd like to call it). Note the 'Field ID' for the field as we'll use it in the script.
2) Create the other fields you want for IT and HR and then go to Admin Menu->Field Layout->Field Order and create sections for them (IT first and then HR).
3) Go to Admin Menu->General Setup->Customize Strings->All Strings tab and edit the sNewBugFooter string. Add the following script and then save changes (note that where it says "field1" you should write "fieldX" where X is the ID number of your Type field):
Example 1:
/* How to hide/show fields based on a dropdown value */
/* How to hide/show a bunch of fields based on a checkbox */