Here are some VERY general guidelines from a non-teacher who can't quote sources but just observes.
Use the present perfect if:
1) It is very recent news: The Mets have won the World Series! (Said as the team is jumping up and down on the field.)
2) It is connected to the present conversation/situation: Have you fed the dogs? Yes, I've fed them.
3) It was started in the past and continues to this moment: I've worked here (or I"ve been working here) since June.
4) It was at a non-defined time in the past and it could still happen again: Oh, I've eaten at that restaurant. It's good.
In general, if it can no longer happen again, and/or has no connecting to the current situation use the simple past.
The restaurant is now closed: Oh, I ate there once It was good. Too bad it closed.)
I now work somewhere else: I worked there.
The person is dead: I knew him for 25 years
I no longer travel: I went to France, but I never got to see the Louvre
EDITED to change "past" to "present" - written in error and hopefully without causing too much confusion.