The language of Chaucer and Malory, Middle English can be surprisingly approachable for modern English speakers even 800 years later (although knowing a little French or German doesn't hurt). Let's dive in!
To get started there's the venerable Middle English Dictionary and the Historical Thesaurus of English. The Anglo-Norman Dictionary might also come in handy.
For texts to read there's The Electronic Canterbury Tales and the Malory Project's facsimile editions of Le Morte Darthur.
For a really deep dive there's the Middle English Texts Series, collecting hundreds of texts, often with substantial introductions and links.
There's also the Digital Index of Middle English Verse, which "brings together in highly-structured, interconnected, and approachable form the information scholars need regarding subject matter, authorship, dates, manuscript transmission, and poetic form of all surviving Middle English verse from c. 1200 to c. 1550, including that found in manuscripts, incunabula, inscriptions, wall paintings, monuments, etc." The DIMEV only contains partial transcripts, but there is usually enough information to find a complete copy elsewhere online.
The ongoing Middle English Grammar Project's corpus includes transcriptions of hundreds of Middle English texts, many of them from legal documents, government records, and medical and scientific works.
Finally, linguistics nerds may appreciate the linguistic atlases of Early Middle English and Late Medieval English, together spanning the entire Middle English period.