I am seventeen and looking into modeling and an agent told me to slim down a little I am 5'6 at about 125lbs with a lower metabolism and a body fat percentage of about 20 percent Suggestions?

You could start excercising more.. not even that much more, just enough to boost your metabolism a bit. Also if you're not eating healthy, try to cut back on junk food. Together, that should improve your metabolism and help you slim down. (Although 125 pounds at 5'6'' is already pretty slim).