There's like a small black dot in my back tooth. I think its just a small infection. Will it go away by brushing;. Its something that eats away your tooth? Please help. Im only 14

A black dot may indicate the start of a cavity, the black pigmentation is from decaying enammel and plaque colonies (bacteria responsible for tooth decay).

Without a visit to the dentist immediately, then I would suggest thorough cleaning of that spot and supplementation of alcohol mouth wash for a week or so, if the dot is still there or has grown then visit your dentist.