Something you should do after every meal. Most people do it after breakfast and after dinner. Dentists say we should do it more often and then we wouldn't have so many cavities.
To brush your teeth, obtain a specially designed bristled device called a toothbrush (it need not be an electric toothbrush but that can help, or at least be more entertaining.) Put some specially flouridated minty gel (also known as toothpaste) onto the toothbrush, and rub over your teeth.
Note that brushing randomly is not good. Instead, for the front teeth, brush from the gums down for the top teeth, and from the gums upwards for the bottom teeth. The back molars can be rubbed in a circular motion with the toothbrush.
After you have succeeded in brushing your teeth, you may find that flossing will help clean nasty leftover bits of food and plaque from between your teeth.