A deckhand is someone who works on a boat doing the jobs necessary on deck. It has become a very popular job fantasy since reality television swarmed the networks with shows about crab and lobster fishing. Working on a boat is not for everyone, and it is important to remember that.
The first step to becoming a deckhand is to get yourself in shape. Find a great workout routine and stick to it. Being a deckhand requires a lot of physical strength as well as stamina. Cardiovascular activity is recommended to achieve the endurance necessary. Keep in mind that you get minimal downtime while working on the boat, and most of that will be spent eating or sleeping.
The next thing you need to do is scope out locations for fishing. You will have to travel to places where there are major docks, such as Alaska or New England, in order to find work. It's best to figure out which fishing seasons take place on which body of water and when. Also educate yourself as to how long each season lasts so you will know how long you will be away from home. This will be especially helpful when it comes time to pay your bills. You can pay enough in advance, or have someone dedicated to paying them for you, so you still have utilities when you return.
You will have to research what type of boat you want to work on. This will be important because different boats require different jobs, and different jobs require different skills.
In order to legally work on a boat over 100 tons in the United States, you will be required to have a Merchant Marine's Document (MMD). This is issued by the United States Coast Guard and will require a criminal background check as well as a drug test. Obtaining an MMD can take up to 6 months so be prepared to wait. MMD cards must be renewed every 5 years which will require the background check and drug test to be repeated.
The open sea is a crazy place, you never know what kind of weather is going to come at you. It is completely possible that sailing is not possible, and if you don't sail you don't work, which means you don't get paid. So make sure you don't rely on your next job for money. It's a good idea to try and maintain some kind of emergency fund just in case mother nature decides you can't sail.
It's a good idea to join the Seafarers International Union (SIU). They offer training classes and apprenticeship programs to help you get started. They have great resources to help you obtain medical insurance, and get you on your feet.