Tawhid: the oneness of God
The briefest statement of Islamic belief is a whole surah: God is one, is eternal and self-sufficient, does not beget and is not begotten, and has no equal. Its four verses are among the most recited in the Islamic world, and they are the anchor of this lesson.
Around that anchor the Quran builds a consistent vocabulary. Nothing resembles God; the eye does not reach Him while He reaches the seeing; knowledge of what is before and behind creation belongs to Him alone. A reader who accepts these verses has the outline of tawhid: unity of essence, of attributes and of action, without a created thing being compared to Him.
The Shia school counts divine justice among its principles alongside unity, prophecy and the leadership of the Imams, while the practices of religion — prayer, fasting, almsgiving — belong to a different order and are studied separately. Confusing the two is the commonest source of confusion in introductory material, so keep the distinction: belief shapes practice, but a practice is answered in fiqh, not here.
Quran
بِسْمِ ٱللَّهِ ٱلرَّحْمَٰنِ ٱلرَّحِيمِ قُلْ هُوَ ٱللَّهُ أَحَدٌ
Say, "He is Allah, [who is] One,
وَلَمْ يَكُن لَّهُۥ كُفُوًا أَحَدٌۢ
Nor is there to Him any equivalent."
فَاطِرُ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضِ ۚ جَعَلَ لَكُم مِّنْ أَنفُسِكُمْ أَزْوَٰجًۭا وَمِنَ ٱلْأَنْعَٰمِ أَزْوَٰجًۭا ۖ يَذْرَؤُكُمْ فِيهِ ۚ لَيْسَ كَمِثْلِهِۦ شَىْءٌۭ ۖ وَهُوَ ٱلسَّمِيعُ ٱلْبَصِيرُ
[He is] Creator of the heavens and the earth. He has made for you from yourselves, mates, and among the cattle, mates; He multiplies you thereby. There is nothing like unto Him, and He is the Hearing, the Seeing.
ٱللَّهُ لَآ إِلَٰهَ إِلَّا هُوَ ٱلْحَىُّ ٱلْقَيُّومُ ۚ لَا تَأْخُذُهُۥ سِنَةٌۭ وَلَا نَوْمٌۭ ۚ لَّهُۥ مَا فِى ٱلسَّمَٰوَٰتِ وَمَا فِى ٱلْأَرْضِ ۗ مَن ذَا ٱلَّذِى يَشْفَعُ عِندَهُۥٓ إِلَّا بِإِذْنِهِۦ ۚ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ ۖ وَلَا يُحِيطُونَ بِشَىْءٍۢ مِّنْ عِلْمِهِۦٓ إِلَّا بِمَا شَآءَ ۚ وَسِعَ كُرْسِيُّهُ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ ۖ وَلَا يَـُٔودُهُۥ حِفْظُهُمَا ۚ وَهُوَ ٱلْعَلِىُّ ٱلْعَظِيمُ
Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is [presently] before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Kursi extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great.