That's a perfectly valid question! People then learned in much the same way as people do now: through teaching and practice. The majority of medieval 'formal' education would have been within religious communities, and the novices traditionally young boys and girls, but older or more diverse education wasn't unheard of. The British Library recently held an excellent exhibition surrounding the written word in Anglo-Saxon England which featured a number of fascinating artefacts related to your question. Michelle Brown has written an excellent monograph (accessible here) about the continued use of wax tablets in particular throughout the Medieval period as both tools for everyday writing as well as for practising and education. Within the 'formal' educational establishment of a religious community, novices would have been instructed by a master or mistress, and would have learned with the help of dictionaries and phrase books. The 10th Century OE/Latin textbook Ælfric's Colloquy - a source I return to frequently - suggests how classes may have been organised. Some of the exhibits at the British Library illustrate a beautifully human side of past classrooms, from English translations scribbled in between the lines of Latin texts and doodles in the margins, to one particularly enterprising novice who jotted down the opening stanza to Beowulf on a blank space in his Latin translations, presumably to practice his handwriting or spelling.

Classroom education wasn't the only means of learning. Teaching may have been carried out at home, either passed down by parents or by a tutor, likely a cleric. Asser's Vita Ælfredi states that by the 880s, Alfred of Wessex required all of his Ealdormen, or at the very least their sons, to be sufficiently literate and numerate to carry out the bureaucracy of their counties themselves. It's also been suggested that, given its importance as a port city and trading hub, that some 50% of later Medieval London was literate in English, and some 40% in Latin.