To add additional Fields you should follow ntoziers guide located on his website @ (http://www.tmib.net/adding-custom-text-fields-tickets)
As for deleting fields from the open ticket menu that you do not use... I would simply comment them out in case you ever wish to add them back in. For newly created staff tickets the file is located at osticketrootfolder\include\staff\ticket-open.inc.php
I personally use Notepad++ to modify/edit my osTicket files. You can find that program @ (http://notepad-plus-plus.org)
Cheers~
Update:
You beat me by 1 min ntozier!! -haha