You need to keep a record of every phone call/text he/she makes. Keep it on your phone, do not erase it. Keep a log of events such as them driving by your home, following you, etc. You need to take this with you to court to ask for a restraining order. If you feel scared you need to call the police to make a report. That also is good documentation of the stalking.
Text this back, " I got GPS and your location and within a few minutes the cops should be arriving, stocker" it worked for me. You can also call 411 and let them know that this number keeps calling your phone and that you would like to know if they can tell you who it belongs to
Never reply to an unwanted text. Make note of the number, and then block it so that they cannot continue to contact you. Then let your parents or other trusted adult know what happened, so they can take care of it.