The Book of Jeremiah begins by setting the scene—it says that these are the words of Jeremiah, the son of the priest Hilkiah, and that he began to prophesy during the thirteenth year of King Josiah's reign, (which would be around 626 BCE).
The word of the Lord continued to come to Jeremiah through Josiah's reign and those of the kings after him, until King Zedekiah was defeated by the Babylonians and Jerusalem was captured by the Babylonians in 587 BCE. This is one prophetic marathon.
God's word first came to Jeremiah when he was a boy, telling him that God had known him before he was born and destined him to be a prophet.
Boy Jeremiah protests that he's just a kid, but God tells him not to be afraid. He'll send him to different people, commanding him to prophesy different things, but he'll always have his back.
God touches Jeremiah's mouth with his own hand, putting his divine word into Jeremiah's mouth. He makes Jeremiah someone who can speak truth to power, and can prophesy both destruction to the nations, and their eventual re-building.
God shows Jeremiah two visions: the branch of an almond tree, and a boiling pot tilting away from the north.
God helpfully explains the symbolism of the last image to Jeremiah: he's going to send an army of tribes from the north (meaning the Babylonians) to besiege Jerusalem and conquer it.
But Jeremiah will need to be like a fortified city or pillar because the people won't like it. He needs to speak whatever God commands, and can't crack or give into their demands.
God says that people will oppose Jeremiah, but God will save and sustain him. We'd guess J's getting a little nervous just about now.