Install iTunes on your main computer before trying to use your iPhone/iPad/iTouch.
The Nozbe app syncs task automatically with Nozbe on your iPhone, iPad, or iTouch. Get it in the App Store (click the icon on your iPhone/iPad/iTouch desktop).
Make sure the Nozbe iPhone app syncs well for you before committing to this option, since there have been reports of syncing problems. See here for helpful discussions: https://www.nozbe.com/page/forum?q=iphone+sync.
No syncing is necessary if you access Nozbe via your built-in Web browser at nozbe.mobi.