There are a lot of researches done on the history of Berlin Wall construction. If you are writing some paper on it, you’d better perform a thorough research on it.
Still, if you are only wondering, I will try to explain it shortly.
The Berlin Wall construction began on August 13, 1961. It was built by GDR (German Democratic Republic) to separate the West Berlin from East Berlin (as the Germany was also divided into 2 parts - Eastern and Western, for it was occupied and controlled by different countries). There were a lot of people trying to escape the Communist Repression and flee the country to reunite with their families. So the wall was kind of holding them inside the country.
Also, one of the reasons to build the wall was to keep the spies out. The Soviet Union was very suspicious and did not want any information to be transferred to potential spies from other countries, as it was the high time of a Cold War and the Soviet Union wanted to beat the United States. Also, the separation of a country had a bad impact on the daily life of people. Eastern Berliners had faced a lot of daily troubles, they became dissatisfied with life, for the usual services were disappearing, less and fewer people stayed, and there appeared a higher demand for certain jobs. So the situation occurred when there were too many positions and no people. The wall seemed to be a solution for a growing crisis.
So, if we list the main events that led to a Berlin Wall construction, we should mention the following: