Install iTunes on your main computer before trying to use your iPhone/iPad/iTouch.
The Toodledo app syncs tasks automatically with Toodledo. Get the Toodledo app in the App Store (click the icon on your iPhone/iPad/iTouch desktop). Synchronization instructions are found here: www.toodledo.com/info/help.php.
No syncing is necessary if you access your Toodledo tasks at www.toodledo.com/slim. However, this requires an active cellular data connection.