If I miss a period when will the next one come?!

Not necessarily. If you're missing periods because you are young and still irregular, it could come at any time. I know, it sucks.

Not an expert, but the same thing happened to me so i asked my gynecologist about it. She told me that if you do not have your period (and are not pregnant) that means that you did not ovulate that month. She said this is common and nothing to be too concerned with. If, however, it is repeated months, i need to take a medication to "jump start" my period. I would recommend getting a gynecological exam. They can determine if anything is actually wrong. Hope this helps!