All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
What the brother advised you that the supplications should be recited after the obligatory Prayer and before the Sunnah Prayer is correct. According to the Sunnah, when the praying person finishes his Prayer, he should utter before changing his sitting position: Astaghfiru Allah (I seek the forgiveness of Allah) thrice. After that, if he is an Imam, it is desirable for him to face the worshipers who are led in Prayer. Then, he may recite other supplications proved by the Prophet Muhammad .
For more benefit, please refer to Fatwa 84021.
Moreover, we advise you to seek true Islamic knowledge and try your best to increase your religious knowledge in order to be able to worship Allah as the Prophet has taught us. You may get good knowledge by referring to authentic books and from trustworthy Muslim scholars as well.
Allah knows best.