i would describe it as:
メ=ma in may
ア= ah like when you open your mouth and say "aaahh"
リ= this is the hardest one. some people say that the "r" in japanese is like a mixture between r and l. "i" sounds like "ee" in "see."
ー= this sign just means that you extend the vowel sound that it followed, in this case it's "i." I find it easiest to think of it in terms of music: if "ri" is a quarter note, then the "ー" is like another quarter note. Together "リー" is a half note.