I want to know how to be steadfast and what to do to achieve that?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The road to Allah starts by avoiding His prohibitions and abiding by His Orders, thus knowing that the person was not created vainly and uselessly. Allah Says (interpretation of meaning): {"Did you think that We had created you in play (without any purpose), and that you would not be brought back to Us?"} [25: 115].
In fact, he was created to fulfill a great mission: worshipping Allah. Allah Says (interpretation of meaning): {And I (Allâh) created not the Jinns and humans except they should worship Me (Alone).} [51: 56].
The person has then to free himself from the servitude to any but Allah. He should also know that his lifetime is short and that the pleasures of prohibitions come to an end while their sins stay written in ones individual account. So, the person should perform all duties and do as much Nawfil (supererogatory acts) as he can. Allah Says in the Hadith Qudusi reported by al-Bukhari : "And the most beloved things with which My slave comes nearer to Me, is what I have enjoined upon him; and My slave keeps on coming closer to Me through performing Nawafil (praying or doing extra deeds besides what is obligatory) till I love him" .
So, dear brother, choose a good company and avoid bad people as the person usually follows the religion of his/her best friend. Again, feel sorry and deep sorrow for the sins you committed previously. Try to make yourself conscious of the enormity of the sin, as compared to the Greatness of the One you disobeyed.
It is said: "Do not look at the smallness of the sin but consider the Greatness of the One you disobeyed".
Allah knows best.