You may have a list of possible topics. To narrow these down:
Make sure your topic fits into one of the IB specified subject areas.
Choose a topic that will keep you interested for a long period of time.
Try not to pick a topic that you have strong personal feelings about as you may not remain open-minded.
Consider if there is a teacher who can supervise this topic.
Make sure you have the resources you need to carry out your research.
Consider choosing a topic which relates to your future career aspirations.
(https://www.futurelearn.com/courses/ib-extended-essay/7/steps/669241)
Let’s take the example of a student who is exploring the topic of how people learn new languages.
They may initially generate a question such as “What impacts students’ ability to learn English as a second language?”. However, this is not specific enough. A more effective title might be: “What cross-linguistic influence is experienced by Hong Kong children learning English as their second language?”. The original question was improved by:
using a specific case study rather than general terms and concepts
narrowing the scope by focussing on a specific geographical area
narrowing the scope by focussing on a particular demographic.
Chemistry: What is the effect of decreasing the temperature on the reaction rate and rate constant of fading of photochromic dye?
Visual Arts: To what extent can the painting 'Grain of Sand' by Abdul Mati Klarwein, be considered as religious art within the context of his installation; 'Aleph Sanctuary'?