I have grown an apple tree from seed.
Apples are generally hardy and therefore it is safe to plant the young tree outside.
They require a well drained, but moist, soil and a warm open position away from shade and frost pockets. I would recommend moving the young tree to a cool but light position over the winter ready for planting out in the spring. Try to do this before the plant comes into leaf in order to avoid problems with frost tender foliage. Once you have chosen a location dig a hole twice the size of the pot and mix the excavated soil with compost and a good handful of slow release fertiliser. Backfill the hole ensuring that the young plant is firmed in well and keep well watered until established.