I went on two cruises when I was a teenager with family (father, former step-mother, and former step-sister). Despite being with two people I didn't care for, it was a pretty good time. Both of mine were via Carnival ships. As a teenage boy, I was probably interested in different things than will attract you but it was still a ton of fun. I can tell you that snorkeling in the Caribbean was pretty cool. One time was flat out amazing. Great visibility with a wide variety of beautiful fish, coral, and other oceanic life to see. Otherwise we didn't participate in too many excursions. We would often just get off the ship and walk the markets because the former step-mother had more interest in shopping than other activities... much to my despair. That said, be wary of any deals that seem too good to be true in those markets. Fake designer products and jewelry is rather common. Also do keep in mind what they say about drinking the water in Mexico is true and extends to the ice unless it's blended into an alcoholic beverage. The alcohol seems to kill whatever causes the problem but otherwise it will not be fun.
Aboard the ship itself, there are so many things to do. Between random activities, shows, the casino, places to dance, pools, exercise areas, sun decks, shops, spas and just about anything else you could want there's really no reason to ever be bored. The other thing about being on a cruise is there is almost constantly some place where you can grab something to eat. I know Carnival ships have (or at least used to) a 24/7 Pizza place at the back of the ship (with nice salads too). It was like being in a completely different world. Keep in mind that's the entire point and take advantage of opportunities that attract you.
As far as boarding and all that stuff, I don't remember anything being particularly special about it. Reboarding after an excursion required a passport or birth certificate... not just a cruise card. Unboarding at the end of one of the cruises was delayed because of an illness issue. You'll have to do muster training at some point where you're instructed where your muster station is in the event of an emergency and some basics on the flotation devices. Otherwise, you can almost forget you're even on a ship at times.
Have fun!