A Muslim is a person who believes in only one God, Allah, who created the entire universe, and that Muhammad (PBUH) is his final messenger on earth.

The basis of any Muslim starts with the firm belief in the oneness of Allah SWT. Allah is the alone Almighty, creator, and sustainer of every living thing in this Universe. He begets nor is he begotten. 

With being a Muslim there are also  Six Basic Beliefs as laid out in the Hadith and Quran:

• Belief in the Oneness of Allah Almighty: Allah says in Quran: “Say: He, Allah, is One.” (Quran, 112:1). And He says: Say: “He is only one God, and surely I am clear of that which you set up (with Him).” (Quran, 6:19)
• Belief in Angels: Muslims believe in unseen beings who worship Allah and carry out His orders throughout the universe.
• Belief in the Books of Allah Almighty: Muslims believe that Allah revealed Holy Books to a number of His Messengers. These include the Quran, the Torah, the Gospel, the Psalms, and the Scrolls. The other revealed books were revealed before the Quran, this is because the teachings of the other revelations are included in the Quran thus making the Quran the final revelation.
• Belief in the Prophets or Messengers of Allah SWT: Muslims believe that Allah’s guidance has been revealed to humankind through Messengers, or Prophets.
• Belief in the Day of Judgment (Qiyamah): Day of Resurrection is believed to be God's final assessment of humanity.
• Belief in the Divine Decree: The belief that Allah SWT knows everything and that He knew everything about His creation even before He brought them into being. The Qur’an says, “He is Allah, other than Whom there is no other god, the Knower of the Invisible and the Visible.” (Soorat Al-Hashr, 59:22)