My ex is trying to harass me. I dont know what to do?

I think you should report him to the authorities. If there are any, any adult would do, really, if you are a teenager. If you are older than a teen, then, I think you should probably call the cops on him or her, the person has no right. I had someone like that too, he tried to rape me, I told the school and though it did not necessarily go my way, he never got the chance to. Get help if he or she is harassing you, raise an alarm, call them out, they will not harass you again. And above all, stay alert and safe.