Now, you all will probably jump right on me for this suggestion...
But for someone at that height, with that weight...I say eat whatever you feel like my man. If you feel like pizza, eat pizza. You know yourself better than us, and if you truly do have a crazy fast metabolism then I say go on a "see food" diet...eat whatever you see, for the most part. If you start to gain too much fat because you realize you've been eating sh*ttier than you should have, start to incorporate some of the stricter diet advice given here or hell just do a bit of cardio. BUT make sure you're training the right way, heavy compound lifts, focus on the bench squat and dead (if you can with your knees and back). Go heavy, look up Bill Starr's 5x5, it works wonders for many people new to lifting. Unfortunately it focuses around the squat and dead, and if you can't do these correctly then don't do them at all for risk of injury.
I'm not advocating you drink regular soda or swallow a gallon of Hagen Daaz every night. Try to keep it clean but honestly, I wouldn't worry about being strict. You need calories, you're 20 years old, and you can't keep weight on. If you have the ability, the time or the need to get stricter with your diet, the suggestions listed so far in this thread have been excellent.
You have a low BMI (21.4). For me at 6'0, I'd have to weigh 154 lbs...I haven't been less than 200lbs since I was 16. That is LIGHT.
Eat whatever, just make sure you eat enough.
Best of luck.
...Go ahead gentlemen, tell me why I'm dumb. :bruce3: