If you've been growing it in the house you'll need to harden off by putting out during the day and bringing in at night for a couple of weeks, then it can be planted in the ground. Be aware that your tree will not necessarily be anything like the apple pip you grew it from and as it hasn't been grafted on to a dwarf or slow growing rootstock is likely to get very large.

Just to add that if you do plant it outside place a screen around it to keep animals from munching on its tender sweet bark and leaves. Once it gets bigger you can place a protective wrap around the trunk for a while serving the same purpose.

Most apples need another one nearby as a pollinator. If you get yours to flower and there are any other apples nearby what I did was to beg a spray of flowers and put them in a jar at the foot of the tree - it worked...