I have a new computer how do i de authorize a computer i am no longer using

Individual computers can only be deauthorised directly on them (via the Store > Deauthorise This Computer menu option), but if you have 2 or more computers authorised then you can log into your account on your computer's iTunes via the Store > View Account menu option and 'deauthorise all' (which you can do once every 12 months) and you can then authorise/reauthorise the computers that you still have and need : authorising and deauthorising.