I'm 18 years old male, and I'm very very scared that I have colon cancer. I got some symptoms, but no blood. Thin stools, 1st time dark green stool today. Really scared.

So rare. You are more likely to get hit by a bus today than have colon cancer at your age. I'm not sure what symptoms other than "no blood" you refer to, as most colon cancers have no symptoms. If you are having abdominal pains, fullness, nausea, etc - then you should see doctor as there are literally hundreds of conditions other than cancer that could cause symptoms in abdomen.
Unlikely. In the absence of a strong family history of cancer or other rare medical conditions, colon cancer would be one of the last things your doctor would consider when presented with the described symptoms. Benign causes such as alteration in diet, effect of certain medications, and functional bowel disorder are more likely, it would be advisable to consult your doctor if the problem has been ongoing.