depends on what engine as most are now have auto tensioners so you can't really get it wrong.
First step to do if you've never done this before is buy the Haynes book for your car (£15 at halfords) and read the section first before doing ANYTHING. Cocking this up can lead to expensive engine rebuild costs.
tip: before removeing the belt, ensure all the cogs line up with the TDC mark. That way you can't go wrong.