The correction of Lokmane is true, I will try to complet it.
Rencontrer = To meet for the first time
Trouver = To find and Retrouver = To find again
We use "retrouver" in different cases :
1- J'ai retrouvé mes clés = I found my keys.
2- "Je retrouve mes amis à 9h" means you have a meeting at 9 with tour friends. I think in english you say "I meet my friends at 9 o'clock" or "I see my friends at 9 o'clock".
3- You can say : "J'ai retrouvé mon veil ami" which means you meet your old friend who you didn't see since ages. I don't' know if you can say in english "I found my old friend".
I hope I was clear, I'm sorry for my bad english!