Belief in Allaah is the firm acceptance of the existence of Allaah. It is true confession that He is the One and has no partners; that there is nothing which resembles Him or equals Him; that He is perfect in His qualities and attributes, free of any defects; and that He creates and controls universe. Therefore, He is the only One worthy of being worshipped. Worshipping anything else is shirk.The essence of belief in Allaah is the acceptance of the existence of Allah and his oneness in subtance/esence and character/attribute and worthiness of being worshipped and obeyed.
Belief in Allaah makes man realize that he is created by Allaah and will meet Him after death. He will count his deeds and will reward him as good as he do. He will submit his life and will be afraid of him only.
We realize our belief in Allaah by declaring an uttering the two testimonies (No one is worthy of worship except Allaah and Muhammad is His messenger), beleiving in our heart, and performing the obligations and refraining from the prohibition, adhering to the Islamic manners.