I ate it = past simple
I have eaten it = present perfect
We use both tenses to talk about past events and actions - the difference is that when we use the past simple, we are talking about a completed or finished action in the past, we can use a past time expression when using past simple - eg., I ate fish for dinner yesterday.
When we use present perfect we are speaking in general - we are not speaking about a specific time in the past - we do not use past time expressions with present perfect - eg., I have eaten it - sometime in the past I ate it but it's not important to say when.
Conversations often start with present perfect and then move into past simple when the speakers wants more information.
example:a) Have you ever eaten salmon?
b) Yes I have eaten it once before but I didn't like it.
a) Oh, really, when did you eat it?
b) It was about a year ago, I was on holiday.
Notice how the dialogue begins with a general question and then moves into past simple when the speaker asks for specific details.
I hope this helps :D