Medieval Studies is only available as a Minor Plan. Therefore, you must choose a Major Plan to accompany it.
ENGL 100/6.0 (see first year English)
HIST 121/6.0 or HIST 122/6.0 (History: see What HIST course should I take?)
LATN 110/6.0 or GREK 112/6.0 or ARAB 100/6.0 (see Languages in First Year)
Of these three, Latin is the best prerequisite for upper-year courses related to Medieval Studies
For a list of electives see here