How to help someone being bullied:
Stand up for the person being bullied. Think about how you would feel if you were the one being bullied. Would you want someone to stand up for you? Bullies look for those that are alone. They think by intimidation, no one else with step-in. By standing up for the person being bullied, you are showing the bully and the one being bullied that they are not alone.
Don't give bullying an audience. Instead of laughting or supporting it, you are letting the bully know it's not entertaining.
Help them get away. You can create a distraction or offer a way for the bully to leave the scene. Only intervene if it feels safe to do so.
Tell a trusted adult. An adult can help stop bullying by intervening and can help by giving the victim support.
Be their friend. Being friendly to someone can go a long way in letting them know they are not alone.
Set a good example. Don't bully and others may follow your example.