Paraphrasing the Summa: First part, Question 3

Question 3. The simplicity of God

Is God a simple or complex being?

The Catholic Church teaches that God is pure spirit.  Ancient mythologies would picture their gods as immortal humans or animals.  So, it is important to that those who believe in God understand what God is and is not, in order that they don't fall into the error of worshipping the creature instead of the Creator.

Catechism of the Catholic Church #43 Admittedly, in speaking about God like this, our language is using human modes of expression; nevertheless it really does attain to God himself, though unable to express him in his infinite simplicity. Likewise, we must recall that "between Creator and creature no similitude can be expressed without implying an even greater dissimilitude"; and that "concerning God, we cannot grasp what he is, but only what he is not, and how other beings stand in relation to him."

To investigate Question 3 in more detail, St. Thomas breaks it down into the following areas:

1. Is God a body?
2. Is He composed of matter and form?
3. Is there composition of quiddity, essence or nature, and subject in Him?
4. Is He composed of essence and existence?
5. Is He composed of genus and difference?
6. Is He composed of subject and accident?
7. Is He in any way composite, or wholly simple?
8. Does He enter into composition with other things?

Is God a body?

No.  God is pure spirit.  But what is spirit?  Spirit is sometimes called ether.  Sort of a cloudlike, formless, essence.  It is also called "immaterial", which simply means that it is not any sort of material.

Is He composed of matter and form?

The Church says that God is spirit.  So, no, He is not composed of any matter nor does He have any form.

Is there composition of quiddity, essence or nature, and subject in Him?

I don't understand any word in this question, except, "composition".  And we know that God is not "made".  Composition implies being made and also, being made in different parts.  God is spirit and is eternal.  He is not made nor is He composed of any thing.  

Is He composed of essence and existence?
Is He composed of genus and difference?
Is He composed of subject and accident?
Is He in any way composite, or wholly simple?
Does He enter into composition with other things?

All of these questions have the word, "composed" or "composite" in them.  Therefore, we know that the answer is no.  God is utterly simple.  He is not composed of anything.  He is pure spirit.

Well, that's the way I understand the questions.  Let's see how close I came to the answers.