The Distance variable makes no sense (as explained above). Distance to what?
Finding the n closest balls of a ball is a well known problem, asked several times in the old forum.

You can use brute force if the balls aren't too numerous, comparing the distance of each ball to the other balls. There are tricks to reduce the number of comparisons by avoiding to compare to itself, to balls having already checked the distance with the current ball; and by avoiding to compute the square root of the distance formula since we want only to compare, not to get real values.
You can then compare the current squared distance to the last smallest you stored, and if even smaller, store it to the biggest stored.

Another way, more complex, is to use a partition of the plane, to compare only to balls in the same region. It is called quadtree (IIRC) and there might even have a library for that.