How to get % crit percentage

[RaulIgnat]

So how much crit % would be enough ? is 100% rly needed ?
With a value of lets say 50% crit , how much actual chance of a crit do you have for your first hit ? and for 2nd of 1 does not crit ? 3rd ?
Im quite used to exploiting this from game like dota so i am curious how i can use it here im my favor.