The related links page provides useful information and resources specific to your country.
If you feel like you are in immediate danger, contact your local police straight away.
Blocking or deleting the person from your contact list is another good thing to do, but make sure your parents or a trusted adult is aware of what has happened too.
It is important that you tell someone what has happened if you are made to feel uncomfortable online.
The first thing you should do is tell an adult that you trust. You can also use the Report Abuse button, which will send an online form to the police.