As a person who started learning German recently, I would recommend you don't worry about this for now; I don't know Arabic, so I don't know if there are any similar language features, but I do speak very good Spanish and English, and German cases and declining are not in either of those languages (there is something similar, but it's not a perfect equivalent). With that said, not knowing this when you start learning won't hold up your learning in any way, in fact I think it's quite the opposite; focusing too much on them right away might slow you down. Just wait a little bit, get a feel for the language; they'll eventually make sense to you, in a weird way, you won't be able to explain them, but you'll feel them. Trust me :), I've been learning for 10 months, I don't quite "feel" them yet, but I do notice a improvement every day!

First, a pronoun is used instead of a noun, actually in order not to repeat it. So, the question is more when to use the four cases, either as a noun or a pronoun. The nominative case is used for the subject, the accusative case corresponds more or less to the direct object object in English and the dative case to the indirect object. The genitive case describes a relationship of belonging, translated into English often with 'of'. But more precisely, it depends on every single verb which has its own structure. For example: Kennst du den Mann? Ja, ich kenne ihn. 'du' respectively 'ich' are nominative, 'den Mann and 'ihn' accusative, because 'kennen' (to know) reqires the accusative case. Ich helfe dem Mann/ihm. 'dem Mann/ihm' are dative, because the verb 'helfen' (help) requires the dative case, actually it is hard to tell why. der Hundes des Mannes (of the Mann) would be genitive.