Born in 1432 to Murad II, he was sent at 11 years old to Amasya in northern Turkey with his mother (supposedly a low-born woman), teachers, and advisors to learn how to govern. He also had a particular mentor, Molla Gürâni, who was a scholar, philosopher, and jurist who had a high position in Murad's and Mehmed's administrations. Another influence was Akshamsaddin, a Sunni Muslim scholar, poet, and mystic.
When Mehmed was 12, his father abdicated, leaving Mehmed in charge. Why would he do this? It was June 1444, and Murad had just made peace with Hungary after defeating the Crusade of Varna, an attempt by several European Christian countries to check the growing power of the Ottomans. He had several other victories under his belt, and perhaps wanted a life of leisure.
Hungary under John Hunyadi broke the treaty a few months later—the papal legate, Julian Cesarini, had convinced Hunyadi that breaking a truce with Muslims was not a sin–and Mehmed prepared to fight.
There was a problem: the Janissaries. Although loyal to the sultan, they were uncertain of this pre-teen who was now their commander. Their unrest caused Mehmed to ask his 40-year-old father to return and lead the armies. Murad's response was "You are the sultan; you lead the army." Mehmed's reply: "If you are the sultan, come and lead your armies. If I am the sultan, I hereby order you to come and lead my armies."
Mehmed retained the title of sultan and continued to govern, but Murad returned as commander of the military, defeating Hunyadi (again) and retaining authority until his death in 1451, when Mehmed became sole ruler for the second time. He was almost 20 years old, however, had much more experience, and was ready to take on the world.
His first goal? Constantinople. See you tomorrow.