It is great to find that Allah gave you his favors and blessing at this young age. I pray to Allah that you become one of the great scholars of Islam.
As for this atheist, you have to first prove to her that this perfection you see around you in the trees, oceans, animals, insect etc can only be created by a Wise, Knowledgeable and Powerful Creator.
If we bring a pile of wood and leave it in a room, can it ever turn into a table and chairs? They will say that this is impossible and there has to be a carpenter to make such a thing.
Likewise, this perfect universe can only be created by the Creator. Once she agrees to that, you can go on to step two.