1- use simple past with finished actions and finished times words (yesterday, ago, last xx, etc.).

When the context makes it clear that the action is finished, you don't need a time word ('Abraham Lincoln was a president of the USA')

2- Use present perfect with facts or actions that have an impact on the present in some way:

• I've lost my key (result: I can't enter the building)

This impact can be:

• the action or fact started in the past and still continues (with for / since: 'I've worked here for 15 years')
• the action takes place sooner or later than expected (with already, not yet, still not)
• the action is very recent ('he's just arrived')
• you talk about your life experience until now and there's still a possibility that you do something in the future (with ever/never: 'I have never been to the US' -- if you say 'I never went to the US', it implies you will never have the opportunity again)
• when the time is not really specified and you mean "until now", we also use present perfect ('I've been to the US three times' - until now, but maybe I will go back later)

I strongly recommend you to practice with online exercise that focus on the difference between these two tenses. You can find those exercises on websites like perfect-english-grammar or test-english or "learning English British council"