when I turned 18 my periods changed,,,

Hi,
Welcome to ehealthforum,
The long gap between periods could suggest anovulatory cycles (menstrual cycles without periodic and regular egg release from ovary), or causes like PCOD, ovarian problems like ovarian suppressions, or failure, hormonal derangements, thyroid hormone imbalances, anemia, etc. The exact cause can only be made out after careful evaluation. Consider visiting your doctor/gynecologist for thorough examination and further evaluation to look for the underlying cause for the absent periods. Treatment as needed (based on examination findings and investigations results) would be started to help you get back your periods and regulate them.
PCOS/PCOD usually responds to treatment with insulin modifiers as most common cause for PCOS is insulin resistance. If you have been diagnosed as having PCOS/PCOD, consider discussing with your treating doctor/gynecologist about getting started on these along with hormonal contraceptives for better control of PCOS and for regularization of periods. You can also consider discussing your pregnancy plans with your treating gynecologist. Once the PCOS is under control, ovulation inducers would be started to help you have more than 1 egg getting matured to increase chances of pregnancy. If you are trying for pregnancy, monitor cervical mucus changes and the BBT (basal body temperature) charting to predict your ovulation (along with the follicular study) and time your intercourse for having fruitful results. Have intercourse in positions that allow for deeper penetration for high deposit of semen. Have diet with foods rich in zinc content to ensure better maturation of egg and sperms. Take adequate rest. Drink plenty of water. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.