If you've never tanked before at all then you absolutely can't go wrong with a Dark Knight. Grit + Unleash spam with generous methods of replenishing your mana (one combo attack, + two different cooldowns) make it nearly impossible to fail unless you're deliberately going out of your way to be bad. Unleash doesn't have to be targeted and has an enormous circumference. It's perfect for leveling tanks with all the more difficult minutia (maximizing dps mainly) saved for end game in harder content.