The Perfect tense is used, when we're refering to something that happened in the past. This tense is mainly used with spoken German. With reports or news, we use the past tense.

This is the 'fight' between Perfekt (ich habe gegessen) and Präteritum (ich aß). You'll find a lot of answers starting from here: http://www.italki.com/question/255398

In addition to these answers, you should also know that the preterite and the present perfect mean the exact same thing. There's no distinction in German between something continuing to happen and already being completed. Also, even in spoken language, you use preterite with some common verbs like wissen (wussten), denken (dachten), all modal verbs (wollen, müssen, dürfen...ect), sein (waren), and haben (hatten). Also, I recommend that you pay some attention to German grammar. When I was a beginner, it was only after I knew grammar relatively well that I began to pick up vocabulary quickly because I could actually USE the new words if I knew how to. Let me know if you have more questions.

Präteritum is equal to the English simple past: I ate = Ich aß Perfekt is equal to the English past perfect: I have eaten = Ich habe gegessen. However, the use of these two tenses are different in English in German. German uses the Perfekt more in spoken German while English uses Präteritum more in spoken English.