Learn your tawheed (and it’s opposite; the shirk) more than anything else. For whoever dies as a mushrik is destined to abide in Hell, for eternity!
Shirk is the only sin that has it’s unique place in the Qur’ān. In fact, many scholars are of the opinion that there is no single ayah in the Noble Qur’ān except that it is related to tahweed.
إِنَّهُۥ مَن يُشْرِكْ بِٱللَّهِ فَقَدْ حَرَّمَ ٱللَّهُ عَلَيْهِ ٱلْجَنَّةَ وَمَأْوَىٰهُ ٱلنَّارُ
Indeed, he who associates others with Allāh – Allāh has forbidden him Paradise, and his refuge is the Fire [Surah al-Ma’ida 72]
Any sin that is lessor than shirk is subject to Allāh’s ﷻ will, if He ﷻ wants He will forgive it. Shirk, however, is different.
إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُ ۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدْ ضَلَّ ضَلَٰلًۢا بَعِيدًا
Indeed, Allāh does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allāh has certainly gone far astray. [Surah an-Nisaa 116]
Do not think that just because you are now a Muslim, you can no longer fall in to committing shirk. This is indeed a trap of shaytaan, he lets you feel safe. No one feels safe except for the wrong-doers.
There are many types of shirk that are most commonly practiced in our societies, while people think they are safe. For example, democracy, taweez (read more here 1 and here 2), horoscope and many others.
I will give you a short guideline on where to start.
1. Qur’ān — You cannot learn tawheed and refrain from shirk, better than from the book of Allāh ﷻ
2. Kitaab at-Tawheed by Sheikh Muhammad Ibn Abdul Wahhab — A very short and to the point book compiled to address common categories of shirk (purely based on Qur’ān and authentic ahaadith)
3. Four Principles of Shirk by Sheikh Muhammad Ibn Abdul Wahhab — A very concise book to address 4 types of shirk where sheikh discusses why is shirk of today is worse than the mushrikeen of Makkah
4. Fundamentals of Tawheed by Dr. Bilal Philips — Concise book purely written in English to address very fundamentals of tawheed and it’s opposite
There are a lot of other books but these ones are the most important ones especially for English readers.