An Extended Essay consists of the following components:
Choose a topic that is meaningful to you. It should be in a subject you are currently studying. This makes sure that you will use the language, philosophies, and methodologies from that subject correctly. It is also a good idea to write your essay using a subject you are familiar with because you will have easier access the content and sources you need. You do not need to choose a subject that you think you will be studying in university, although that does give you and your teachers more to write about when you apply to a school.
In any case, the subject and the topic you choose should lead you to a "workable research question". This means that you have found a question with a clear focus that can be successfully answered within the 4000 word limit.
Here's an example of how you can narrow down a subject and topic that will lead to a workable research question: