Anno 1800 is all about finding new land to take over. If you really want to build a bustling civilization, then you will need to find new islands to colonize. A big part of this is Fertility. Islands can only grow certain crops, so you will need to find new places to grow the various things you need to keep your population happy.
How To Colonize A New Island
To colonize a new island, you first need to find one to colonize. Select your boat, and start exploring the map. You should, ideally, be doing this from the start of the game. As you discover islands check to see if they are inhabited. This is pretty easy as buildings tend to stand out. After that, check the Fertility of the island by looking at the icons above the minimap when your camera is looking at the island.
When you find an island that is both uninhabited and possess the fertility for the crops you need, it is time to colonize it. To do this, you need to have 10 Planks and 8 Steel Beams in your ship. You will also need $2500, so make sure you have enough money before you set off.
Find a beach on the island, and as long as you have all the resources you need, you will be able to build a Small Trading Post there. Note, you will not be able to build a small trading post on an island that is controlled by another player.
Now, simply build the trading post, and you will have a new island to start exploring! You can either build up a new city that can provide for itself or a specialized production island that just makes a resource you need. You can also ferry good between both islands, in case one can produce something that the other needs.