How do you determine when your about to get your period?

If you've menstruated before then you can determine when you're about to get your period based on your average menstrual cycle length - this is normally around 28 days but everyone is slightly different.

Otherwise it's a little trickier to predict, everyone's bodies change leading up to menstruation - for example about two weeks prior to your period you will ovulate so your temperature will change and you will likely get fertile cervical mucus (an egg-white type discharge), after that you may notice your discharge thicken, changes to your mood, skin, breasts, etc. Many women don't learn to recognise these changes until they've menstruated for a while.