What even marked the beginning of the French Revolution?

There were two. The first was the storming of the Bastille on July 14th 1789. The second was the Tennis Court Oath.

When Louis XVI had to convoke the Estates General, the third estate (the bourgeoisie) ordered to double their numbers in the Estates General, because the majority of France existed of Bourgeoisie. The others 2 estates (the nobility and the clergy) did not agree with this, but the King gave in to the third estate. When the Estates General met, they had on one occasion locked out the third estates members. They went to a nearby tennis court at Versailles and swore they would not dissemble until a constitution for France would be drawn up and accepted. Both occasions were significant for the beginning of the French Revolution.