Think about the methodology first!
Find the mathematical methods or techniques that you are interested in. THEN think about some topics you want to apply those to.
You need to be deeply understanding the method or technique, so take a lot of time researching and learning those.
Then, brainstorm the ways that you can apply those methods and techniques to investigate different topics or applications that you are interested in.
After you come up with 2 or 3 topics, you can get help from your teacher for choosing the one that would best fit the purpose of IA.
Your exploration is much more likely to be successful - and gratifying - if it focuses on a mathematical topic in which you have a genuine interest, and for which you are confident that you can discuss and use the relevant mathematics in a manner that demonstrates thorough knowledge and understanding. There is no single approach for determining an exploration topic that is guaranteed to be successful for all students. However, often times, starting with a sufficiently narrow topic is more successful than starting with a very broad topic that requires a significant effort to reduce to the extent that it can be explored in less than 20 pages (double-spaced).