It all depends on what you want. Bulk? Lean? Losing Weight?
At your age if you just wanna lose that summer weight, diet is the lazy way to go about it, i won't say it's easy because mentally most people struggle with this, just drop your simple carb content down preferably to non existent, eat veggies everyday and enjoy healthy fats and protein. Your body will go into ketosis and you'll be looking like your old self in a few months time. If your used to eating lots of sugary stuff your body will take a few weeks to adjust and you may very well be pretty miserable during that time.
If you wanna get muscle mass and lose fat and your willing to work for it, there's no better way than to start lifting. There's different ways to go about this depending if you want a leaner look or a bulky look. The really important thing is to not be lazy and hit good muscle fatigue in your work outs.
Cardio is great for improving your Stamina and helping you attain a leaner look, you can get a bit bulky too if you do it certain ways plus go insane with it.. If your one of those guys that enjoys running around then do this, you can get good results from Plyo, Combat etc style work outs.
There are so many proven routines out there you can do, if i went into detail i'd just be describing something somebody else already has. The important thing is showing up and sticking with something.
I've been through quite a few programs, P90x, Insanity, etc. Even used to do the whole jogging thing at one point. Those are fun, but for me, these days i just keep my diet clean and hit the gym 3 times a week and hit every muscle group lifting, i'm big into squats and deadlifts cause they hit so many muscles. Very simple and not very time consuming. Keeps my muscles toned and the fat off. Some people will try to tell you things, like never to hit all muscle groups in a workout. I'll tell you to experiment and do what works for you.