In German we have the Nominative, Akkusativ, Dativ and Genitiv. These are all the personal pronouns but in different cases.
Whenever we start a sentence, and the pronoun performs the action it will be in the nominativ form.
If the action is performed on you or to you it will be either dativ or akkusativ. It depends on the prepositions or verbs you are using.
Some verbs need the Akkusativ and some need the Dativ. Some prepositions may also change the case.
The cases are very important because we have to decline the adjectives and nouns as well accordingly. The best way to learn the German cases is to practice, practice, practice.
Don't freak out, it is not as hard as it seems to be. It is all a matter of doing exercises and memorizing which verbs need to be used with dativ and akkusativ, prepositions ;)