I am 25 years old and underneath my eyes are dark? What can I do to get rid of it?  

Not much. There can be many different reasons for this, genetics, dehydration, severe weight loss (causing fat pads in the eyelids to shrink), smoking. I"d see your family doctor to see if treating the secondary causes may help.
Dark circles. Dark circles under the eyes can be due to lack of sleep, allergies, age, or many other causes. Treatment of allergies if present and getting adequate rest can help some. Cool compresses 2 times a day and makeup coverup can be effective as well. If there is volume loss associated with dark circles, facial filler injections can be considered. Laser skin tightening can be effective in some cases.