I wish I knew what the exact procedures were for a 16 year old, but even if the rules for international players do not apply in the Dominican Republic, things must still be close to the same.
The Gulf Coast League starts their season in June, so sometime before then, you will come to the United States. Also because you are still considered underage in the US, you might live with an American family. That would make things feel more like home.
It is possible that you start in the Gulf Coast League, and then move up to Class A before the end of the season. The Nationals might want to keep some kind of schedule for the first year, then once you are 18, they can move you up to higher levels as you are ready.
You will have to be patient, because there are a lot of good young players in the minor leagues. Because you are younger than most of them, you will have to work harder too. But that is a good thing, because hard working baseball players are more visible, so that means the organization will see you in a better light.
If you don't mind, I would like to give you a little help for when you want to reply to a message. At the bottom of each page, just below the last message, there are 2 buttons. The first one is to start a New Topic, which places your message in a different area. The second button says Post Reply. That second button is the one you should click on when you wish to reply to a message you are reading. When we click on the Post Reply button, it is the same as if we are in a conversation.
I hope you come here and talk to us often. Nationals fans will be watching you as you progress and get better. One day you will be signing an autograph and the person will say to you, "I am The Chief", or "I am Tomterp from the Nationals Fan Forum" and it will be like meeting a friend you have known, but have not seen for a long time.