We've already seen in class the difference between these two tenses:
I've known Peter for 20 years, since we were at school
I met his at primary school, when we were small children
If you want to extend the explanation given you can go to this link about present perfect; you'll find a large grammar reference and some exercises at the bottom of that page.
Here you are some links to practise just in case you still have problems (I'm afraid two of these are repeted from the previous link; check it, please) :
See you in class.