Study with a university introduction guide to Medieval English. Many universities, such as California State University and Harvard, offer introductory lessons on pronunciation, vocabulary, alphabet and history. They often start with the alphabet and basic pronunciation and move on to more complex concepts. These lessons will be the most valuable, and affordable, beginner's option.
Read some material written in Medieval English. Geoffrey Chaucer is the most famous writer of this style of English. Regularly reading Medieval English with pronunciation in mind will help with memory, new vocabulary and can be inspirational in the learning process. Read out loud to a friend for even more practice.
Use an online Medieval English dictionary to gain more vocabulary. Medieval departments at universities will often have an electronic dictionary to look up unknown words. These sites can help bolster your vocabulary and pronunciation while you're reading works in Medieval English.