Congratulations on starting a tree from seed. You may not get a tree that bears fruit however as most commercially grown apples are from grafted trees and the seed from the fruit may not breed true or be exactly like the mother. It is fun to see what will happen so I suggest you transplant your seedling outdoors in a sunny location soon. The brown on the leaves could be sunburn but don't worry as the tree will build up a "tan" and become used to full sun. Remove the brown leaves as they form in case it is a leaf blight that could spread. Once outdoors, with good air circulation any fungal diseases are less likely. Keep growing, Marianne Binetti