If its a mini apple tree, it won't reach orchard size anyway, so it should be fine, and remain small, in the ground. Best dug up and transplanted in November or December, so long as the ground isn't frozen.

You've asked how to encourage it to fruit - prepare the area you're going to plant it in by digging it over and adding some composted animal manure (from the garden centre). Dig a hole wide and deep enough to accommodate the roots as they are, without bending or crushing them in anyway. Then back fill with the soil you took out, tread down firmly round the base. If you have chosen an exposed situation, you may need to stake the tree at planting time.
As it grows, it should fruit well, but you must keep an eye out for the various problems which apple trees get, such as codling moth etc., and ensure you keep it well and frequently watered for its first two years, while it settles in, particularly once any fruitlets have formed.