Once you have made a snapchat account, go into your settings, and click on "who can send me snaps".
Make sure it is set to friends only!
If Someone is annoying you, or bullying you, consider blocking them by going into the block tab.
I Hope this is helpful, and just remember if it gets really serious, then go straight to the police! this is just a guide, so you can have basic net safety and be safe!