If the horse is green, you need to start with a snaffle bit, not a curb. The reason why is that your green horse most likely does not neck rein (which is how curbs are ridden with), and a curb doesn't offer 'single rein' pressure like a snaffle bit does. Your horse is also not going to be ready for all the extra pressure on his poll and chin, that a curb bit distributes, as he's not going to understand why it's there...you are likely to cause alot of head flinging, and possibly rearing, if you simply start with a curb bit.
Then teach neck reining if your Western by putting a rope loop around his neck. When you apply direct pressure to the bit, also pull the rope one way or anther so he gets the hang of neck reining. When you think he's ready, move him to a curb