If it is your first time, I would take .5mg just like T. Calderone stated. Perhaps even 1mg, but remember - the less is better because you can always take more but can't ever take less.
Besides, a first time user taking a 2mg bar will most likely have you pass out/black out fairly quickly, do stupid things you will possibly regret and have zero recollection of any of it.
Be safe and start low. Work your way up, but be careful. Benzo addiction is NO joke. Moderation is the key.