use it

When you speak about something in the present perfect you say that
it is not finished,
it is not over yet.

I have lived here since 2001 (and I still do).
She has studied Latin for three years (and she still does).
We have been friends for a long time (and we still are).

Maybe the action itself is over,
but the time period is not finished yet.

I have met him twice this week. (still same week)
Today she has invited me to her party. (still same day)
We haven't met him yet. (still true)

The present perfect is about the past (that's when it started)
and also about the present (because it is still true now).
The simple past is only about the past (it started in the past, and it ended in the past).

Compare:
We were best friends (but not anymore) and We have been best friends (and we still are).
He lived here until 2015 (but not anymore) and She has lived here for many years (and she still does).

In many sentences there are clues that tell you:
use the present perfect

Words like:

this week (month, year...), today,
for years (months, weeks, a long time...),
since March (last Monday, 2000 ...),
already, before, yet, lately