My period is irregular?

I had my 1st period on mothers day and I've been waiting for my 2nd one but i. Am a week over and I'm not even on the same track as my sisters. I'm scared that I'm going to start it while I'm on vacation cause I'm going to be so cautioned. I am NOT and never been sexually active. So I'm not prego. Is this normal? How long will it be til I get my period again? Will it always be this irregular? Help!!!???!!!!