The Existence (Wujūd, وُجود) is usually the first necessary attribute to know because its primary signification is the existence of the Divine essence itself. Obviously this is the most important aspect to know first. Why? Because all other attributes of Allah exist by virtue of it.
Wujud is not an eternal attribute that describes the essence like the human whose existence is separate to the existence of his body, rather, it is a term merely used to describe the presence of a Creator.
Existence for a thing is either a rational necessity, an impossibility, or a possibility. This attribute is rationally necessary because the intellect can never fathom the non-existence of Allah.
Background
Proving the existence of Allah is the major topic in the modern/post-modern discourse. This topic was considered a marginal issue centuries ago, as most of the world normatively accepted theism, but today, with the entrenchment of Darwinian socialism, scientism, positivism, and the similar like of post-Enlightenment ideologies, it is perhaps the most discoursed topic in the field of theology, across all remaining monotheistic religions.
The rational evidences are plentiful from classical works to modern works that prove the existence of Allah from multiple angles. Most evidences are built upon each other using modern understanding of hard sciences in mathematics and physics, or bringing the discourse up to modern nomenclature.
Arguements
The two most often cited arguments for existence of Allah are rationally based from the Quran itself. They are the Argument for the Contingency of the World and the Argument for Intelligent Design.
There are other arguments as mentioned below:
The Moral Argument: This argument asserts that objective moral values exist and can only be grounded in a transcendent being, which is identified as God. If objective moral values exist, then God must exist as their source.
The Ontological Argument: Originally formulated by Anselm of Canterbury, this argument proposes that the very concept of God as the greatest conceivable being implies His existence. If God exists in the mind, then He must exist in reality, as existence in reality is greater than existence in the mind alone.
The Argument from Religious Experience: This argument claims that personal experiences of the divine or numinous are evidence of God’s existence. While subjective, these experiences are presented as universally transcendent and warrant consideration as evidence.
The Argument from Reason: This argument suggests that human reason and the ability to comprehend abstract truths are best explained by the existence of a rational source, which is God. The reliability of our cognitive faculties is argued to point to a divine creator.
The Argument from Beauty: This suggests that the existence of beauty in the world points to a divine creator. The appreciation of beauty and the presence of aesthetic values are seen as evidence of a source beyond the material world