The first important event that was part of the Reformation (as opposed to a cause of it) was Martin Luther's posting of his 95 Theses. This happened in 1517. This was Luther's first very overt attack on the doctrines of the Church.
From there, the next important event was the Leipzig Debate in July of 1519. During that debate, the arguments of Johann Eck, Luther's opponent, forced Luther to go so far as to challenge the authority of the pope. This had been implicit in his 95 Theses, but now he had to say it openly. This break with the Church continued with his publishing of three pamphlets in 1520.
Finally, there was Luther's appearance at the Diet of Worms in 1521. There, Luther refused to recant his beliefs and was made an outlaw. This led to him being protected by Frederick the Wise of Saxony. This was the beginning of the situation in which some German states supported the Reformation and became Lutheran while others remained Catholic.
There were, of course, many more important events in various countries that are part of the Reformation. However, these are the events that got the Reformation started.