The present perfect shows a connection between an action that happened in the past (represented by the verb in the past participle) with a present situation (represented by the verb has/have). Observe this example:
I have bought a car.
It means that I bought a car (in the past) and I still have it (in the present).
In case you specify a time in the past when an action took place, you simply use the simple past. Example:
I bought a car last week. (last week marks the time in the past when the action happened.)