sorry, but that's a very complex grammar issue.

most of the time you're using Dativ if you can ask "Whom?" (Wem?), but there are some words (prepositions) that require Dativ just beause (but I doubt you're handling those words, since you said it's your second year).

You should read other articles on it, maybe there's a wikipedia page in your native language, do the exercise you got from your teacher and learn together with your classmates ;)

Hope that helps =)