Welcome to the forum, 16 is not to late to start boxing.
Would look into ABA gyms in your area if you would like to compete.

Beginner and Advanced classes are usually separated, with the beginners class focuses much more on technique and skill development, while advanced is more focused on fitness.
Why not go to both, if you are serious about training you need to be going more than once a week.

Foundation is important, you need to focus on footwork in the beginning.
Running is excellent way to improve fitness, so you should go running when not in the gym.
If you are able to skip at home, this can be a great way to improve conditioning. Try to skip for 10-15 minutes in a row, do not worry about skipping rounds.
Nothing stopping you shadow boxing at home, running, skipping, pressups, sit ups, leg raises.
Pay more attention to running.
There are plenty of youtube videos which can teach you about boxing, one I would recommend is

https://www.youtube.com/user/myboxingcoach

Just focus on jab, cross(right hand straight) and left hook(all to the head), while using your footwork.
After you master this, move on the defensive side of boxing.