If you’re US Based, you’ll need to attend college at one of the State Maritime Academies, or the Federal Merchant Marine Academy (yes there are technically other paths, but let’s go with the the easiest and most realistic for a 16year old)
One school you might attend is SUNY Maritime for example, which you can pay for through Scholarships, Grants, and Student Loans, (if you’re a NYS resident you also may have free tuition available to you depending on how the Excelsior scholarship program is rolling out). You should note this path of study is academically rigorous. Meaning it’s kinda tough, make sure you’re a decent student. You also face other pressures that normal college students don’t face.
After you(hopefully) graduate, you’ll have a license as a Merchant Marine Deck Officer, specifically 3rd Mate(Unlimited Tonnage), as well as a degree such as BS Marine Transportation, BS Marine Business and Commerce, BS Marine Environmental Studies, or one of a few other similar degrees of various names. If sailing on a deep sea merchant ship is your goal, you’ll likely want to sail either for Military Sealift Command, which operates all the Navy’s non-combatant vessels. Basically all the supply, fuelling, forward positioning, research, etc ships for the navy. Those ships are all manned by Merchant crews. The other option is to sail out of the Union Hall, such as MMP, MEBA, and AMO. These three unions do the Manning for almost all US Deep Sea shipping. It’s a bit of an old fashioned way of doing hiring. A job comes up, say a billet as 3rd Mate on the Mearsk Denver, and whoever has the most seniority gets the job, if they want it, if not it passes down until someone take it. As a new applicant, fresh out of school, you may wait weeks or even months before your first job, get a side hustle to sustain you, like bartending, or boat repair, or whatever you want.
After your first Job as 3rd Mate aboard a ship, you’ll progress, over years, to 2nd Mate, Chief Mate, and someday Master (the proper term for Captain). That is if you don’t get a job shoreside first. Very few Mariners eventually rise to be a Master, most who go the route I describe sail for a few years and transition to shoreside management in the marine industry.
Some things that will prevent you from becoming a Deck Officer:
Drug use or criminal history. Either of these come up in security and background checks. We also have federally regulated random and pre-employment drug testing, so even if your state legalised pot, you won’t be able to partake.
Medical issues. Including but not limited to Asthma, heart conditions, mental/emotional disorders, etc. basically anything you might need to take medication for. There’s no pharmacies at sea. And the coastguard, who approves your medical info, frowns on any conditions that would impair your fitness to carry out your duties at sea, whether that’s standing navigational watch on the bridge, or responding to an emergency such as a fire aboard.