You start by learning the basics and then adding to that.
Start by reading a short overview of what programming is and how things work, ignoring language.
C++ is a challenging language, and on top of that you will need to add graphics, sound, I.O, and UI elements.
You can learn the basics at
https://www.learncpp.com/
but be prepared to spend a lot of time writing simple console programs at first.
Once you have worked through that site, then you will need to start making decisions on how you will do the graphics and windowing and such.
While you are learning though, you can refine your requirements. For example, what will be in the first person? Most of the popular sea trading games are mostly 2-d, where you control shipping and production and such from a map of the area, with occasional combat (pirates?) in some of them which may or may not be in 3-d or first/third person. As you imagine yourself playing this game, what do you do, what does it look like, .. what parts are first person, what parts are on a map or other interface, ... when is it set? Are you the ship captain or do you have many ships? Write it all down so you can have requirements and a solid idea of what you want to do later. Maybe even play some of the classics if you have not.