I'm 6'2" and when I was your age I weighed about 140 lbs and had the worst time gaining weight, now I weigh about 190 at age 26. That's a 50 pound difference but if you look at the time period that's only about 6 pounds a year so be patient!

My suggestions:

Eat constantly, although try to be reasonably healthy at the same time, eating fast food all the time will make you feel like ****. My suggestion would be a ton of peanut butter and like other people have said, milk is amazing, I drink a half gallon a day. If you're an ectomorph like me you'll have to work harder than everyone else to put on muscle (and weight in general) but you can still achieve great results in the end. I wouldn't worry too much about supplements, you are not trying to avoid carbs or fat so they are relatively pointless except as meal replacements when you can't eat real food. Creatine can be useful as it will generally make you retain extra water weight and can help with muscle building.

As far as training, focus on the large muscle to train they will help with the bulk. Squats of course are excellent as other have stated, I'm a big fan of pushups, pullups, military presses, and deadlifts also. Playing basketball or another sport that involves a lot of running can actually make your legs very powerful without a whole lot of "effort". Working a heavy bag can really build up your chest, back, shoulders, and arms while being fun at the same time.

The main thing to remember though is that it will get progressively easier as you get older and your metabolism slows down! In time, it will become an asset as you will be able to keep the fat off your new muscles much easier than others...