Pbanks13: No one can really know your odds of success - although you can certainly get some insight from others who have been successful. The only 100% guarantee we can tell you is that you won't get a NROTC scholarship if you don't apply!
You will read of people with "better" records who didn't get a scholarship and others with "weaker" records that do. The board's decision is based on your application, your grades, your test scores, academics, demonstrated leadership, essays, interviews, physical fitness test and the needs of the Marine Corps. That last category (needs of the USMC) is the one that can really swing things from year to year.
Keep doing what you are doing. Constantly seek to improve your grades. Take and retake the SAT/ ACT. Look for leadership opportunities in your activities. Work out hard and often. And most importantly...
Start working on your application as soon as it becomes available. Work hard on the essays. Get it submitted as soon as possible. Prepare for your interview (there are some good threads on this subject in this forum). You will take an official Physical Fitness Test as part of the application process - nail it. Then sit back and wait for the results while enjoying your senior year.
Thank you for your interest in serving your country. This alone makes you stand out from many of your peers.