If I get the question right, you want a folder's content to be deleted on boot.
The script could look like this:
rm -r "$folder"/*
To use it you only change the second line: replace /your/folder/here with the folder you want wiped. Then copy/paste it into an empty text file (not using something like libreoffice or Open Office, but with something like gedit). Save it as whateveryouwant.sh. Right click on it, go to "Properties"->"Permissions" and check the box to make it executable.
There are lots of options to run it at system start up. There should even be a graphical tool in Ubuntu to let stuff run at log in, that might be easiest if you don't need it earlier.