Want to know how my body works please I need help

Hi guys I saw my period on the 14th of this month Nd it ended on the 18th of this same month I don’t really know how to use this app i fact I don’t know when I ovulate Nd also dat thing u guys call fertile window I don’t really know how to calculate it please I need help am 17yrs old I had sex on the 4th of this month Nd he came in me but I saw my period but yesterday 28 I bleed again and it stop dat same day please I need help I don’t know what wrong with me Nd now am seeing discharge(cercval mucus)