Hi im eighteen years old and im one month late for my period what could be the cause?

Multiple factors . There are multiple factors for a missed period. These include but are not limited to stress, pregnancy, pituitary masses and others. If you are not pregnant the best thing is just to wait because your menses usually returns. If you have 3 months of no period then you should be evaluated.
Have you had . Sex? If so, could be your pregnant.