You maybe a late developer, don't worry I was 15 when I started. Soon you're going to start, then you'll feel better knowing you have one. Every girl's body is different and like I said you maybe a late developer. Take it from a girl who started hers at 15 , honey you're going to be alright.
I am 18 years old and I haven't started my periods yet? What's the reasons behind this? Guide me please?
Schedule an appointment with a gynecologist. Getting medical advice from a doctor is really your best option. You could be a late bloomer, it could be a DNA thing, if you're sexually active it's totally possible to get pregnant "before your first period" so to speak. Since I'm not a medical professional, there could be any number of reasons I've never even heard about.
Are you very underweight? Do you jog a lot or do another sport, overdo it I mean? Your organism may be putting off your natrual maturity because it isn't getting enough nourishment or you are overdoing physical strain / exercise. Defintiely see a Health Expert (school nurse?)