Since Newport is missing needed fertility to make hemp for linen, you will need to expand your islands at the 1500 vassal level, so in order to do that you will need a small discovery ship with a sailor as the captain. It is in your best interests to do that as soon as it becomes available to you.
Since the most common island sequence seems to be “Newport, Hemp, Herb, Wheat, Grape, Beeswax” it makes sense that you should be looking for a hemp island and the vast majority of players take this approach since linen increases the income from vassals as soon as you begin supplying it to Newport which is done by transporting.
The next island you will need is herb to make beer for Merchants 250. To discover this island you need small discovery ship with capable sailor. The regular sailor cannot find an herb island or medium islands. To see all the island icons and what the different sailors find, view island types in ultrafine.de so you can figure out what size the island you just discovered is in Island Overview. There are more details in that section of ultrafine that cover the numbers and features of islands in greater depth.
The next island is usually wheat and to do that you really need to acquire the ruby island slot in the upper left of Island Overview. Normally you can gain enough rubies through levelling up and other event rewards to get that one or you can get the necessary rubies in the anno homepage shop by clicking on the ruby icon in the upper left of the game screen next to the quest book.
At this point in the game, you are likely running low on space on Newport and moving the wheat production to another island that has wheat fertility is your only option really. If you have medium sized herb or hemp, you undoubtedly have been switching out goat and sheep farms since they are not fertility items.
Grape and Beeswax can only be found by the experienced sailor in a discovery ship. At this point, it is worth noting that the actual ship size of a discovery ship is irrelevant unless you are extremely impatient and want the extra slots for sails. What the discovery ship finds is totally dependent on the captain type so a small ship captained with the appropriate sailor is the least costly way to go.
What is most important to understand is that the deposit make up of all islands combined is what really matters. Island hunting is really all about luck and you cannot wait for the “perfect” island since they do not exist. For example, large islands do not have stone or iron deposits. So when you find a new island, examine the deposits in Island Overview by clicking on the mining cart and then the newly discovered island and decide how they will work with your upcoming needs. Lots of gold deposits are only useful when you have achieved 4000 Imperials since you can do nothing with them until then, for example, so current and near future needs are more important in that decision.
So you may have to learn how to be creative in your trading to make up for shortfalls in the perfect mix of deposits and islands which changes as your grow your population.
There are other approaches to island selection and that is more about how much you can work with trade and how willing you are to acquire ruby island slots. Some players will abandon the hemp island and replace it before getting to Imperials 500 and replace it with wheat. They then buy linen in trade as their exclusive method of supplying that material.
You could also forego a grape island later and not get the benefit of wine for your Imperials or do the same with Beeswax and forego candlesticks for the time being. You can still ascend population by buying the necessary materials on the trade market and pay whatever is being asked at the time.
In Island Hunting, you never really know what you are going to find in the next discovery run, so it is probably best to colonize what you need next no matter what the size and get familiar with how to abandon an island and then replace it with a better one.