If the question is where, the door of the castle church in Wittenberg is correct.
If you're looking for the reason for his action and explanation of the 95 Theses' content and impact, here you go:
Luther's 95 Theses questioned the church's use of indulgences. The official title of Luther's document in 1517, which he put forth as theses for debate, was: <em>Disputation on the Power and Efficacy of Indulgences.</em> Indulgences were papers issued by the authority of the pope that granted people removal of penance owed for their sins. Luther objected not only to the fact that these papers were being peddled for money, but questioned many of the doctrinal foundations behind the use of indulgences.
The 95 Theses met with great resistance by the church hierarchy because Luther was attacking the money-making asset that indulgences had become for the church, and he was challenging the authority of the pope.
But the common people responded with great interest and enthusiasm. Luther's statements in the 95 Theses spoke to the concerns of many average people who felt pressured to pay for indulgences to get themselves or deceased relatives out of the pains of purgatory. Luther challenged the church to preach a more genuine message of real forgiveness through Jesus -- and that gospel emphasis by Luther was something the common people very much wanted to hear.
