Be safe. Be smart.
What is Online Bullying?
Avoid. Aware. Apply.
Aware: You need to know that it isn't your fault! Bullies will repetitively do cruel things for no good reason so you mustn't blame yourself. Put an end to this by saying 'STOP'. It is important to make your position completely clear to the bully. That is, you don't like this treatment being conflicted upon yourself. If all goes in vain, block the bully and make sure that your account is private and protected - do not under any circumstances share your password with anyone, even your closest friends.
Apply: Save all evidence, everything from minor comments to major threats. This can be kept in case things escalate and can be shown to trusted people who can help such as your parents, friends or a trusted adult. You can even consider reporting the issue to school authorities and even the police (with a parent or guardian). It wouldn't hurt to even report all conversations or posts regarding the issue to the online/application service either.
Be a friend, not a bystander. Just watching and forwarding mean messages can reinforce the bully's actions. Support the victim and report the situation.
Be civil to one another even if you dislike the individual. Treat people the way you would like to be treated.