Remark
This is a huge topic. I'm going to help you confine it. In the first place, your time seems to me to be a little off. The formation of the area is dated at about 3 million years ago. The formation of a desert is much more recent.
What is interesting is what did not lead to the desert formation. It wasn't the earth's tilt and it wasn't greenhouse gases. Scientists currently believe that the tectonic plates were responsible for the deserts formation.
Briefly as the plates shifted around, the water covering what is now north Africa retreated and land formed. Sun reflection was decreased and the area became moist and arid at the same time.
That is one possibility. That makes use of everything you have learned about Tectonic Plates.
Cities
You might want to think about cities in the Sahara. There are some, I looked them up. You could discuss them relating them to the direction those in the desert took. Make yourself or find a map of the Sahara. These are the main cities in the region.
Important cities located in the Sahara include Nouakchott, the capital of Mauritania; Tamanrasset, Ouargla, Béchar, Hassi Messaoud, Ghardaïa, and El Oued in Algeria; Timbuktu in Mali; Agadez in Niger; Ghat in Libya; and Faya-Largeau in Chad.
The Sahara is a big place something like 3000 miles across going as far east as Cairo, Egypt and as far south as somewhere near Burkino Faso -- one very rich, one very poor. (Cairo is rich). Tracing the location of these cities on a map should tell you quite a bit about the nature of the desert.
Why is the city there?Who are its trading partners? What do they trade?What is their source of water and food? More on this below.Is there rain? When? Surprising answer to this question.What is the population. What is a city and why do people live in them?
Water
The most important consideration of any natural resource but particularly water is where is it? Remember every person living needs a certain quantity of water daily. Without water we can survive at most 3 days and then some of us experience organ failure.
Footnote: The book of Esther in the Bible records that Esther asked her people to fast (no food no water) for 3 days praying that she would be able to save them. When I read that, I tried it. I lasted 2 days. I could not have done another moment after sundown of the 2nd day. I imagine 4 days really would have done some damage. Those who lived in the Sahara routinely faced that sort of deprivation. Water is a resource that limits greatly how many people can live off it. That's why the major cities are on the edge of the desert for the most part.
Nomads
When you read about the people of the Sahara, you find out that they were nomads. They wandered all over the place. You need to answer a very simple question: why? When you read further, you find out that these nomads have livestock. A goat his no idea what difficulties his master faces. All he knows is that if he does not eat or drink, he dies.
But there is something else that you should make note of. These animals need food and water, just as we do. The nomads have to take that into account. The goat will tell them when they do not have enough. Then the nomad has to move. Their buildings are tents. They cannot build a more permanent structure. If they want to give up their nomadic way of life, they have to go to the well established cities mentioned above. Geography will tell you that the nomads for the most part have traveled east and South.
An extremely important question for you to consider is what is the economy of these cities like and why are they the way they are.
Conclusion
These are questions you might want to consider. Anyone of them will lead to some sort of paper on the Sahara. But you must confine it.
