Here are some tips to use when it happens to you. 

Tip n.1: tell your parents. even if it seems scary you have to tell them because they can seriously help you.

Tip n.2: go to the police if it is realy bad. they can track him/her and follow him/her.

Tip n.3: if you think it's to scary to tell you parents then go to someone else. for example: your theacher, your best friend and your family.

Tip n.4: what ever you do NEVER SEND NUDES!!!