Brushing Teeth Help?

I am 12 years old. Despite the fact that I am now 12 years old a month ago, I had a very bad habit of not brushing my teeth atleast 2 times a day since I was 7 years old. I think my cavities are starting to grow worse and worse. The last time I checked the doctor was in September, whereas the dentist cleaned my teeth. After brushing for 2 times a day for a month I stopped again. Now it is May and when I check my teeth, few of the baby teeth are like filled with black lines.

So the main question I have to ask is should I brush 2 or 3 times a day or see a dentist and ask for help?