Remove anything in the way of the leaning tree. It will fall in the direction it is leaning.
Turn on your chainsaw or pick up your ax.
Make a back cut on the side of the tree that is not leaning toward the ground. The back cut should be about 1/2 the width of the tree. If the tree does not fall after the back cut, make a slightly deeper back cut until the tree does start to fall.
Run in the opposite direction of the falling tree to avoid being hit.