The “み” here is short for “みなよ”, which itself is short for “みなさいよ”.

The “み” is the stem of “みる” meaning “to try”. We see this a lot, so it should be familiar. “なさい” obviously means “do this”, so altogether it’s kind of like “do try”.

Finally the last thing to mention is that the full phrase, “みなさいよ” can sound really bossy (because we’re telling someone to do something using “なさい”).

So casually, or between friends, you should use “みなよ”. Or even more casually, “み”.