It is really important to speak to your parents before using the app so that they are aware about what you are doing and may be witnessing. That gives them the ability to help you if you have a problem.
Make sure your privacy settings are set to allow you to only receive messages from your friends. Although it could be your friends that you are targeted by they are often the ones you trust the most whereas it is dangerous for you to receive a message from a stranger.
Don't lie about your age. When creating your account, provide or real age. if you are under 13 you will be provide with Snapkidz which is a version that is age appropriate and will ensure you stay safe.