Hi ,there irregular periods at around your age nowadays is due to obesity causing hormonal imbalance ,and this is again lifestyle dependent with less activity , intake of junk food etc . Which when left as it is may cause various problems in later Life, therfore since I are very young change sedentary like style and food habits and reduce weight and stay  got  and your Mendes will become regular , but if u are unable to do so then consult  a gynaec

it is your obesity which is causing it. you have to keep your weight under control. your are just eighteen. treatment is not required at this stage. irregular periods are because of anovulation. if you keep your weight under control they would normolise

See some hormonal irregularities is quite common in this age so don't think coz of that your belly fat is increasing.... Indulge yourself in the exercise, proper balance  home made food, avoid junk food, drinks,... If Your weight is properly maintained, hormones will fall in place.... Once check your thyroid hormonal level also

Go to a gynaecologist and get ur hormones checked. Depending on the reports correct medicines can be prescribed. Regular cycles will prevent further weight gain.