Most young horses are light in the mouth at the beginning, it takes a while for people to screw them up. The two mistakes that I think people make the most frequently are...
1) Thinking that light feeling is the horse being on the bit and trying to keep it by not allowing them to pull down into the contact.
2) Being afraid of the young horse and being too tight/holding tension in the reins-which becomes tension in the reins and mouth and neck and shoulders and back etc.
Just remember that for your horse to be "Through" You should be able to ask her to put her head wherever you want. If you want to bend right or bend left it should be possible, if you want her to lower her head it should be no problem. Just keep riding forward into consistent contact and she'll come through over her back for you.