I have to agree with Kevin, that if you really need to ask, then IMHO, you are not prepared to train a young horse.
That being said, any bit with a 1:1 ratio (regular snaffles, whether they are d-ring, full cheek, loose ring). Avoid anything with shanks no matter how many people tell you "They are wonderful for training young horses". Shanked bits are designed for the more broke horse who understands neck reining and being responsive without having to contact the bit very much at all. Shanked bits are designed to be ridden with one hand on a loose rein.