Assalamu Aleikum.
I have never prayed, fasted, payed zakah in my life but I want to become a practising muslim. But what keeps stopping me is that I keep thinking about the major sins I did in the past. And also these past days I've been thinking what if I committed shirk

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.

You should hasten to sincere repentance and know that your Lord is Oft-Forgiving Most-Merciful, and that no matter how grave your sin may be, the Pardon of Allah, The Almighty, is greater, and His Mercy encompasses all things. You should also stop thinking that you have committed Shirk (polytheism) because the basic principle is that you have not, and Allah, The Almighty, will forgive all sins that you have committed as long as you sincerely repent to Him. A sinner who repents of sin is like a person who has never committed a sin. As for the missed prayers and fasting, the majority of the scholars held that it is incumbent on you to make up for all the missed prayers and days of fasting in order for your repentance to be complete because they are considered a debt to Allah, The Exalted, which is more deserving of being paid off.

Nonetheless, you should only make up according to your ability, in a way that does not cause harm to your body or livelihood. The Maaliki scholars held that it is sufficient for you to make up every day for the missed prayers of two days. As for the Zakaah, it becomes due whenever its conditions are met. Among these conditions are that the wealth must amount to the Nisaab (the minimum amount liable for Zakaah), which is equal to approximately eighty-five grams of pure gold, or about 595 grams of pure silver, and must also have been in one’s possession for a complete lunar year without going below the Nisaab. If these conditions are fulfilled, then it is obligatory to pay the Zakaah that you did not pay in the past. You should know that Zakaah does not decrease one’s wealth in the slightest; rather, it increases it. The Prophet said: “Wealth never decreases on account of charity.” [Muslim]

If many years passed without you paying the due Zakaah, then you have to pay the Zakaah for all these years and sincerely repent to Allah, The Almighty, for having withheld it. Be confident in the Pardon of Allah, His vast Mercy, and His immense Generosity and Kindness, and know that He graciously accepts whoever turns to Him in repentance.

Allah Knows best.