when does your cicle starts?

Ok so according to <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">eve</a> my cycle starts on the first day of my period, and after it comes my fertile days... but I though that the 14 day of your cycle you are more positive to get pregnant... my my green days end before my day 14 (if I start counting since the first day of my period)... Help me Im soo confuse🙈