<p>Check out these:</p>
<p><a href="http://www.artofproblemsolving.com/AboutUs/AoPS_A_About.php%5B/url%5D">http://www.artofproblemsolving.com/AboutUs/AoPS_A_About.php</a> (lots of great information here, as well as forums to discuss math)
<a href="http://www.usamts.org/%5B/url%5D">http://www.usamts.org/</a>
<a href="http://en.wikipedia.org/wiki/American_Mathematics_Competitions%5B/url%5D">http://en.wikipedia.org/wiki/American_Mathematics_Competitions</a></p>
<p>Ask your teacher if he/she has the "Art of Problem Solving" books you can use to study.</p>