Your first period may be very light. In addition, after you have your first period, you may not have another one for a couple of months. It's hard to say. If you feel comfortable enough, you might ask Mom how hers were when she first started.
However, Mother Nature is screwy - so ALWAYS have something with you in case your period starts and you are not at home.
If you feel crampy, you can take Aleve / Pamprin or Midol. Pamprin or Midol are made specifically for periods as they help with cramps and bloating (the fat feeling you may have). There may be times during your period that your pants feel tight - that's bloating. Nothing to worry about. It goes away once your period stops.
A heating pad works as well for cramps, if you have one. If you are in your teens and you just started having your period, your periods may be irregular and may become regular when you are in your twenties.
I was 12 when my period started and it always came on the 15th of the month at 4 a.m. Why at 4 a.m.? I have no idea - I would feel it in the middle of the night and have to wake up to take care of things. Very annoying. Welcome to womanhood!