Where do melanoma's moles normally appear?

I had a little search on melanoma (skin cancer) and it said that people with large number of moles have a big risk on having melanoma and I happen to be have LOADS of moles. my question is where do melanomas normally appear? and how much time does it take till I can define if it's a melanoma or a normal mole? I mean it appears first like a normal mole then it changes shape, right. how many time does it take to change shape?