Question
Sir, I have recently been clearing away an overgrowth of brush.  I discovered an apple tree growing in the midst of all this.  I don't as yet know what kind, as I can only surmise that we used to toss apple cores to the squirrels, and that one of them "planted" some seeds.  I should like your advice, on fertilizing, and any pruning that I should do, as I would like to develop this into a healthy fruit bearer.  Thank you.

Answer
Good for you. It will be interesting to see what kind of apple tree you have. Since this is a easy question to ask but a more lengthy one to answer I will direct you to a web link that will give you information on pruning, fertilizing, etc. You can skip down to the pruning since your tree is already planted.

http://ohioline.osu.edu/hyg-fact/1000/1401.html