The first exercise should cover the concept basics, and it should have no prerequisites.
It should provide only enough information to allow the student to solve the exercise at hand.
Many tracks have already implemented an exercise covering basics.
To familiarize with what is commonly taught in such an exercise, we recommend that you go sign up to a few different tracks that have Learning Mode, and solve the first concept exercise.
Each language is different, and you might find that you need to introduce more than other tracks do, or less. For example, if you need to compile a program to get it to work, this is something that you'll want to mention in the introduction to the exercise.
Other things that are often covered within the basics exercise are:
Use as few data types as possible. If you can manage, use only integers, as their notation is the same across most languages and will be well known to virtually everyone.
If there's a track for a language that is similar to yours, go ahead and check if you can fork their exercise and use it as a starting point.
Aim to be concise; you don't want to overload students with information. The introduction should explain just enough that the student can look at the code and identify what the different parts are. They don't need a deep understanding of any of it, but they should also not be left wondering what something is.
The student should only need to actively use one or two concepts.