How to write a reaction that you have never seen?
There is no general method for predicting the products of a chemical reaction, given only the starting materials and no other information.

You will learn certain trends, such as, for example, the increasing reactivity down the Group 1 metals with water, or dilute acids, but this often isn't enough to generalise to more complex situations.

You'll also learn about relative reactivity, that allows you to predict that, for example, although (because zinc is more reactive than copper) zinc reacts with copper sulphate to form zinc sulphate and copper, copper does not react with zinc sulphate in the same way.

Often, even after the products of a reaction are known, it can still take a good deal of research to establish exactly how it took place - how the starting materials interacted to form intermediates, for example, and how those intermediates interacted to form the products.  This is known as the reaction mechanism, and although many reaction mechanisms are known, plenty are not.

You should continue to study, and learn as much as you can about the principles of chemical reactivity, and you'll find that in more and more cases, there are patterns that let you predict what will happen in similar cases.  But there are often exceptions, that are only explainable once you have an in-depth understanding.

There is often no alternative to actually doing the reaction, and seeing if what is predicted actually happens.  This is why chemists do research!