When Frederick Douglass was twelve years old, he was sent by the wife of slaveholder Thomas Auld to live with Auld's brother Hugh and sister-in-law Sophia, in Baltimore, Maryland. Sophia Auld taught Douglass the alphabet and the basics of reading shortly after he arrived. But Hugh found out and admonished his wife for being so careless, and forbade her from giving Douglass any more lessons. So she didn't.

But due to this event, Douglass had enough of an education that he wasn't starting from scratch. Further, the ban on further lessons made him more determined to continue his education. So, he recalled, he made a point of making friends with as many white children in the area as he could, and whenever he would be sent off on an errand, he'd take a book with him (which he must have acquired without permission) and would coax his white friends into giving him further lessons. He would sometimes take bread with him to bribe these friends into giving him these reading lessons. He doesn't give names for fear they might be subject to retribution (Douglass's Narrative was written in 1845, when teaching an enslaved person was still illegal), but he says the friends who were most fruitful in giving these lessons "lived on Philpot Street, near Durgin and Bailey's ship-yard".

After a short time, he was capable enough to start teaching himself. While he was still twelve years old, he acquired a book called The Columbian Orator, and could read it on his own. The book was a collection of speeches and essays, but it was also a textbook used in classroom reading lessons. From there, Douglass had the tools to give himself further lessons.

You can read Douglass's account for yourself on pages 32-39 of The Narrative of the Life of Frederick Douglass.