All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
First of all, we praise Allaah The Almighty for guiding you to Islam, and we ask Him to conclude with it our life and yours.
Undoubtedly, whoever has no faith in the existence of Allaah is a disbeliever rather than a Muslim. If he embraces Islam, no making up for fasting or prayer will be due on him. But if he was a Muslim before he denied the existence of Allaah, he would be considered as an apostate rather than a disbeliever. If he reverts to Islam, the scholars differed as to whether or not it is obligatory upon the apostate (who reverts to Islam) to make up for the missed prayers and fasts. It is obligatory according to the Shaafi‘i and Hanbali scholars of Fiqh, and not obligatory according to the Hanafi and Maaliki scholars of Fiqh.
In our opinion, making up for the missed prayers and fasts is due. So, calculate what you have missed of them, and make it up to be on the safe side and to free yourself from liability.
However, you may make up every day for the missed prayers of two days at least, unless it delays you from earning a living for your dependents, in which case, you may make up every day for the missed prayers of one day only, or as much as you can without harming yourself. For further information, please refer to Fatwa 105182. The same applies to the fasting which you should make up for, even though it is not necessary to be made up consecutively.
It should be known to you that if you sincerely repent, Allaah The Almighty will accept your repentance and change your bad deeds into good deeds, as He The Almighty Says about him who associates partners with Allaah in worship, commits adultery and kills, when he repents and does righteousness (what means):
• {Except for those who repent, believe and do righteous work. For them Allaah will replace their evil deeds with good. And ever is Allaah Forgiving and Merciful.}[Quran 25:70]
• {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."}[Quran 39:53]
Also, it is proven from the Prophet, sallallaahu ‘alayhi wa sallam, that he said: “He, who repents before the sun rises from its setting place, Allaah will accept his repentance.” [Muslim]
So, dear brother, sincerely repent to Allaah, and receive the glad tidings that Allaah will accept your repentance.
Allaah Knows best.