Before you start:
Check to see that you have a tmp directory in your Joomla root directory. If not, create one.
Check to see if its permissions are 755 or 705 and change if necessary
Backup you php.ini file (it is critical to operation of the site and you don't want to break anything.)
Checking the php.ini file for tmp directory settings. Open the php.ini file in an editor and go to the File Uploads section.