Well, first thing you will need to do is try and limit your interest area you wish to study, as math is everywhere!
You must start with this question.
What do you like to do? Do you you enjoy sports, exploring the outdoors, building things, working on the computer, studying the environment, ???
Did you try the topic survey? After answering some yes / no questions about what you like to do, and I believe it is the best way to find topics that fit your interests.http://sciencebuddies.com/science-fair-projects/recommender_register.php
You can also try googling with search topics such as "science fair project ideas math sports."
Since math is everywhere, it is a good idea to narrow the project idea options to areas that you enjoy studying, so that you enjoy your project. Give the topic survey a shot and / or spend a few minutes doing a google search. If you haven't found a topic that peaks your interest, write back with 5 top topic ideas from the topic survey and google search that you might be interested in (or at least interested in that area of study, such as computers or sports), and we'll go from there.