Welcome to the forums and congrats on your new lotty
Firstly you will need to check your lotty tenancy as some sites do not allow trees (they are too permanent a plant, and can get out of control, impacting on future use of the plot itself as well as shading etc neighbours' plots).
Assuming you are allowed them, decide what size tree you want. I'd advise the smallest one for the reasons stated above. Size is determined by rootstock - do not just buy a tree from a supermarket or similar, because you won't be able to predict the rootstock and thus the ultimate size of the tree.
Then decide your variety. Some do better in different parts of the country.
And look around the area for other apple trees as they aren't self-fertile. You need to have ones nearby which blossom at about the same time so as to pollinate the blossom.
