i will show you how to whiten up teeth its pretty simple

1.) press the o button and make shure that the dodge tool is selected then go over the teeth to whiten them up!!

Before

after

but of course there is another way to do this

1.) have the picture open and then press the l button then right clikc on the lasso tool and select the magic lasso then just go around the teeth that need to be whitened and once they are seleceted then go to layer>new fill layer>solid color and change the opacity to about 30% and then select a white color and thne you got white teeth that look like this

if the teeth stilll look too yellow you can change the opacity of the fill layer in the layers pallete by having the fill layer selected and then change the opactity were it says opacity and then you have white teeth!!