The past simple form - 'I met' - is used to describe the past. You need to use this form when you are talking about past time (for example, 'last week', two hours ago, in 2005, when I was a child). The present perfect form - 'I've met' - tells you something about the present time, not the past. For example, let's say you were bored and lonely the last time I spoke to you because you didn't know anybody in the city where you are working. But then you phone me today and say 'I've met some new people' or 'I've made some new friends'. Because of this you are feeling happy now. This is an example of how we use the present perfect. I doesn't matter when this event happened - what is important is that it tells you something about the present time. You're happy now because you've made some new friends.

You only use the verb in the past form ("I verb+ed") to describe the past. You need to learn how to use "I have verb+ed" to move your English ability past elementary level.