Oneness Of Allah
The most important part of Islam is the Oneness of Allah. It means that Allah is one. None shares in His attributes. He is the all Powerful, the all-Knowing, the ever-Living, the Eternal. He posses all the attributes of perfection. It can be also said that He does not posses any attribute of imperfection. So, we do not say that He can be ignorant of something or He can be over-powered by someone else, etc.Chapter 112 of the Quran describes Allah's Oneness
قُلْ هُوَ اللَّهُ أَحَدٌ
اللَّهُ الصَّمَدُ لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُن لَّهُ كُفُوًا أَحَدٌ
Say: He, Allah, is One. Allah is He on Whom all depend.
He begets not, nor is He begotten.
And none is like Him.
The word to note is 'Ahad' This is closely related to the word 'echad' in hewbrew, and also the numericial 1 in arabic 'wahid'. Ahad means One, but it also means Unique and Indivisible. God cannot be divided into three parts and God cannot be a man.
The same word is in the Arabic Bible in the old testament. This is the arabic text of Duetoronomy 6:4
اسمع يا اسرائيل. الرب الهنا رب واحد
As you see, The same word is used, This refutes an arguement that some arab christians claim the Ahad means 'one of' not 'one'
The Three Aspects Of Tawhid
Tawhid Ar-Rububiyyah is the tawhid of Lordship. It pertains to Allah’s right as the Lord, and all that entails–the Creator, the Sustainer.
Tawhid Al-Uloohiyyah is the tawheed of worship. It pertains to Allah’s right to be worshipped, without any partners.
Tawhid Al-Asmaa wal Sifat is the tawheed of Allah’s names and attributes–such as that he is Ar-Rahman and Ar-Rahim.