What happens if I don't know how to do one of the essential parts of prayer?

I (14 years old) have just found out about the 14 pillars of prayer, and I know I can do 13 of them but one of them I don't know. I have read that not doing one of them can invalidate the prayer. I don't know how to do the dua in the last rakat (allah-umma sali a'la...). Does this mean all the prayers I ever did were invalid? I feel like I am going to end up in Jahannam and that I have just wasted my life.