Yup, you can. If you are having trouble getting a girl, then that's because of the low chance of getting a girl - every starter has a 87.5% chance of being a boy, leaving the remaining 12.5% to being a girl. So it is a bit annoying getting a girl. However, you can just turn off your game before choosing your starter (saving first, of course) and opening the game again and choosing. If it's a boy, turn off and start over.