Hi OP, this is one of the most popular questions in this sub, so to get you started, I'll link a few previous answers:

• /u/bayoris walks us backwards through time in How far back in time could you go, before modern English becomes unintelligible to inhabitants of the British Isles?

• /u/bloodswan zeroes in on the Great Vowel Shift in When I hear Chaucer spoken in Middle English, it almost sounds more like Dutch than English. But though Shakespeare is closer in time to Chaucer than the present, his English is much easier to understand. What happened to English in between, and why?

• /u/cdesmoulins gives an overview & provides lots o' links to more in Could a modern english speak with a person from the medieval in england?