First of all, choose the right time and the right people. Identify places where you feel safe and comfortable to express yourself. The people around you are also important: they need to be available to welcome you and listen to you, and you need to trust them. They could be some of your friends, certain adults, at school, at home, etc.
Depending on the emotion you feel, you might use different ways of expressing it. For example, if you’re angry, you probably want to let off steam: punching a cushion, yelling into a pillow, going out running, listening to music, and dancing are all good ways of expressing yourself. On the other hand, if you’re sad, you might have a greater need for time and comfort. You may want to cry, write about what’s making you sad, talk to someone, or do things to pamper yourself (take a bath, watch a favourite TV show, listen to or make music, see friends, etc.).