Start with this Du'a
The best du'a, on the Day of Arafah
قَالَ النَّبِيُّ صلى الله عليه وسلم: خَيْرُ الدُّعَاءِ دُعَاءُ يَوْمِ عَرَفَةَ، وَخَيْرُ مَا قُلْتُ أَنَا وَالنَّبيُّونَ مِنْ قَبْلِي: لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
Qala-n-Nabiyyu salla-llahu 'alayhi wa sallam: Khayru-d-du'a'i du'a'u yawmi 'Arafah, wa khayru ma qultu ana wan-nabiyyuna min qabli: La ilaha illa-llahu wahdahu la sharika lah, lahu-l-mulku wa lahu-l-hamdu wa huwa 'ala kulli shay'in qadir
“The Prophet ﷺ said: The best du'a is the du'a of the Day of Arafah, and the best of what I and the prophets before me have said is: None has the right to be worshipped but Allah alone, without partner. His is the dominion and His is the praise, and He has power over all things.”
Jami at-Tirmidhi 3585 (hasan per al-Albani; weak per Darussalam)
Repeat this dhikr often on the Day of Arafah, together with your own du'a.