My advice would be to pick a Paladin or a Druid first to see if you like being a healer or a tank (since both classes can be both) before taking up something like a Priest or a Shaman that can only be DPS/Heals or a Warrior/DK since they can only be Tank/DPS since you're looking for a Tank or Healer. So if you don't like being a Paladin healer you can just respec and try being a Pally tank.
That way you can see if you like being a tank or a healer with one character, then if you like tanking but not Pally tanking you can try DK, Warrior or Druid tanking to see if you like those.