1- If you remember that you had an orgasm such that semen came out, then you must do ghusl. If you didn’t you must repeat your prayers.
2- If you have a doubt whether semen came out or not, then you are not required to do ghusl.
3- Read this Du’a that was taught by Imam Ali (a) to reduce wet dreams:
اللَّهُمَّ إِنِّي أَعُوذُ بِكَ مِنَ الِاحْتِلَامِ وَ مِنْ سُوءِ الْأَحْلَامِ ، وَ أَنْ يَلْعَبَ بِيَ الشَّيْطَانُ فِي الْيَقَظَةِ وَ الْمَنَامِ