"Ami" is a male friend, and "amie" is a female friend (which it looks like you know already!) The trick is: if "ma/ta/sa" ever appear in front of a vowel they become "mon/ton/son".

So in both cases you would say "mon ami" and "mon amie" but if you break it up by an adjective, you would get something like: "mon grand ami" and "ma grande amie"