17 years old, and serotonin deficient... Help/Advice please?

I talked to my science teacher the other day about how I am always craving foods like dark chocolate and coffee. I feel like all the time there is this little voice in my head telling me how worthless I am, and I can go through days/weeks of feeling depressed. I'm only 17 years old - by a couple of months, and my teacher told me I am serotonin deficient. What does this mean, and are there any ways I can help this without prescription drugs? I have found that whenever I feel depressed music always helps me - to a certain extent, and I sometimes just have to go lay outside under the sun, or just feel the wind. I know that sunlight can help, but I don't understand how craving dark chocolate and caffeine can be factors in determining that I'm serotonin deficient. I am constantly tired, no matter how much sleep I get. I feel like my head and neck are in constant pain - but I don't know if this is because of how I sleep or what. I just want to get rid of this feeling of... well... I feel like I just can't do anything. Please help me, or just explain why it is I'm like this. There has to be other people out there like me, and I don't know why I am like this at the age of 17. I need something, or someone to let me know that this can be helped... Please.