How can I improve my mental health?

I used to think that I was smart, however recently I have been thinking about my life, and I realize now that I am not very intelligent.

I feel that if I am able to improve my ability to think clearly, and also my memory, I will be able to accomplish much more than I do now.

I want to know how I can train my brain to become more intelligent, and if it is even possible.

Are certain foods and beverages good to drink for improving mental health?

What about green tea? I heard that it improves your mind. Is that true?

I also think that I may have some minor mental retardation. I have no exceptionally strong proof, but there have been some minor things that might indicate that I have some problem with the brain.

For example I have OCD, but was able to stop doing the 'rituals' by doing Cognitive Behavioral Therapy. Also I tend to be very hyper a lot, and I am not sure why.

If anyone has any ideas on how to improve my mental health, please say so.
Thanks,
Kyle
P.S. I'm a 15 year old boy

Here are four important things you can do to increase your intelligence.
First, stay in school and take challenging classes. Education is related to intelligence.
Second, engage in regular aerobic exercise. Aerobic exercise improves circulation and improves cognitive performance at all ages.
Third, take care of your health more generally. There aren't really any magic foods, but good nutrition, good sleep, and avoidance of toxins keep your brain healthy.
Finally, try new things. Even as a teenager, you can fall into the trap of only doing the things you are good at. Don't let yourself avoid learning new skills.

More importantly, however, intelligence is only a part of the issue. IQ is the best predictor we have of school performance, but it still only explain about half of the differences between kids in their grades. You can make up for average or below average brain power by being hard-working, open to new ideas, and conscientious. This is even more true about other areas in life, like work or relationship success.

Finally, you don't write like someone who is mentally retarded, and I have a hard time imagining that you are and you have never been diagnosed by your school. The OCD part of you is a worrier. Worry can actually interfere with concentration and memory. Let it go. Work hard, be healthy, enjoy your life, and don't worry too much about how smart you are. You'll do just fine.