How do I save my plant?

I've had my tree for 2 years and it seems like through the last year and a half the tree is barely holding on to its life, leaves are more yellow then green. and only has about ten leaves now, how do I save my plant

What kind of tree is it? Different species of trees have different needs, and depending on what type of tree yours is, will depend on how best to go about helping it. Also, is it a deciduous tree, or an evergreen?

are you watering it too much? too little?
is it in the sun?