The fastest and best way to make money is to always fill your ship to it's limit. Empty space is lost potential profits.

Step 1 is to do missions until you have access to the trade journal. I would recommend taking the lightest missions or the lowest crate count missions. If you are in a small starter ship try to keep the weight below 600 to save any trouble at sea

Step 2 is to trade!
Simply put you can just find the cargo with the highest profit to the destination, buy and ship that.

BUT

Something overlooked is that the highest profit margin in the trade journal is not the only metric you should consider

eg cargoes
Copper is expensive & heavy so you can only take so much of it.
Coconuts are cheap, light & bulky so space is the limiting factor.
Dates are super small boxes and light so you could take loads of them.

eg prices
Copper (260 weight): buy 300 | sell 500 | profit 200
Coconuts (50 weight): buy 50 | sell 100 | profit 50
Dates (4.6 weight): buy 40 | sell 50 | profit 10

Copper wins right? well.....

1000 money's worth
3 copper | 780 weight | 600 profit
20 coconuts | 1000 weight | 1000 profit
25 dates | 115 weight | 250 profit

So coconuts win right? well....

1000 weight worth
4 copper | 1040 weight | 800 profit
20 coconuts | 1000 weight | 1000 profit
217 dates | 998 weight | 2170 profit

Okay so dates now? well....

approximately 4 big crates worth of space
~8 copper | 2080 weight | 1600 profit
~8 coconuts | 400 weight | 400 profit
~20 dates | 92 weight | 200 profit

This is a lot, but that's what I love about this game.
Your optimal cargo is based on your money and your ship's capacity

It's all about your situation

If you are starting out with a small ship and not much money I would highly suggest finding the cargoes that are selling for double their buying prices so you can make the most profit per investment.

Once you have the money to fill your ship you need to start worrying about weight and profit per crate.

In larger ships you can really exploit buying large quantities of cheaper cargoes to double your investment every trip!