Install iTunes on your main computer before trying to use your iPhone/iPad/iTouch.
Syncing tasks on an iPhone, iPad, or iTouch depends on which app you have installed. (These devices do not have suitable built-in task lists, so you need an app.) See Priacta’s list of TRO-capable apps at stg.priacta.com/gtd#tro, but be sure to look for an app that syncs your desktop task list, and visit the app publisher’s web site for exact syncing instructions. Share your findings in the comments below so everyone can benefit.