Is this planted in the garden soil or is it in a pot? Whatever the answer, miniature trees are always very difficult and have to be treated with great care, which includes regular watering and feeding. They are far less vigorous than 'normal' apples as they have a special weak rootstock.
Pruning largely depends on how you want to grow them, as 'step over' or espaliers etc. It also depends on variety and whether they are tip bearers or not. Do you know the name of the variety?

Bertiefox is right. You do have to fuss over these smaller fruit trees. They need to be managed. They need a pollinator in many cases. Type the name into google+ pollinator. If your tree is not self fertile you may need to plant another tree that suits your tree and tother way round. Keep watered, fed, and try not to let them overcrop too, as the roots can't support heavy crops. Take off blossom at white bud stage if there is a lot of it. Or wait for the set and thin them out after the June drop. Summer prune extensions beyond fruit in late July. Winter prune to train to shape/height you want in February.