I just bought an iPad an Im pressing the power button an it appears only a cabble with the words "iTunes "

Have you downloaded iTunes to your computer and connected your iPad to your computer with the supplied cable ?

See Here > http://www.apple.com/support/ipad/getstarted/

It depends upon what version of iOS is on the iPad - if it has iOS 4 (which it sounds as though as it has) then it needs to be activated by connecting it to a computer's iTunes (getting started) ; if it has iOS 5 on it then you shouldn't need a computer, you should be able to set it up and activate it directly on the iPad by following the prompts on it.

If you haven't got a computer then do you know somebody who has who would let you activate the iPad on it ?