Okay, first of all, land trade IS possible, but I spend so little time at peace with neighbors that I would not build up trade facilities based on that.

For trade to work at all, you need to have one or more trade buildings, and a chain of ships in every zone to one or more foreign ports. You cannot build a trade merchant where there are no goods to trade, but different goods are worth different amounts. I have not researched this. Once you have a trade building up, you can right click on it to see how much it is producing, and use that to decide whether an upgrade is worthwhile. Other than study the various goods, I don't know how to predict the value of a building. I may or may not study that someday, but I haven't yet.

So what you have to do first is build a Keep in a coastal province, then you can construct a Shipbuilder. The shipbuilder allows you to make little boats to move in the sea zones. Then you need a coastal province with a port and a merchant building. As you make more ships, you extend your line of ships out to the world. As long as no zone is interrupted (by being empty of your ships or the presence of ships from factions at war with you,) your trade buildings with ports will generate income, based on the number of ports you don't own that your ships can reach.

Now, trade is a fragile thing, as you can see. Storms and hostile navies can happen, or everyone can just declare war on you and cut you off. This transient nature has discouraged me from studying it too hard, I just go for extending naval power for spying and opening amphibious attack possibilities, and if it looks like trade might work out too, then okay.