They are just different cases, and to be able to use the correctly, I'm afraid you have to learn the verb patterns. In general, the accusative case is used for the direct object and the dative case for the indirect object: "Der Junge gibt dem Mädchen ein Buch". Also, for location, one uses in most cases a preposition ("in" or "auf") and the dative case. For a direction, it's preposition + accusative case. "Heute gehe ich in die Schule" (in + Akk., "towards school"). "Ich habe gestern mein Buch in der Schule vergessen" (in + Dat, "in school"). But there are many exceptions, you really need to learn the appropriate case and preposition(s) together with the verb.

Hello, it depends on the verbs, which one you have to use. The verb define if you need Dativ or Akkusativ. Unfortunately there is no general rule, you just have to learn it... Most of the german verbs need an Akkusativ,for example: Ich kenne DEN Arzt. Some verbs need Akkusativ and Dativ, for example: "Reichst du MIR bitte DEN Zucker?" Just a few need only Dativ, for example: "Ich gebe IHM einen Kuss". It depends on the prepositions, that you use, as well. You can find the list of the verbs and the prepositions in the internet!

In many cases, a suitable criteria for use of dativ resp. accusativ ist the question word with refer to the object. "Wem" ==> dativ Ich gebe _dem Bruder_ das Spielzeug. "Wen?" or "Was?" ==> accusativ. Wir wollen heute _meinen Vater_ besuchen. Heute Mittag essen wir _den Salat_. With refer to adverbial construction of location the question word is helpfull too: "Wo"? ==> dativ Der Radiergummi liegt in _der Schublade_. "Wohin" ==> accusativ Wir gehen heute _ins Theater_ (= in das Theater) / in _die Oper_.

It helps to remember that the accusative is often directed against something, transitive (you accuse someone), and the dative case is about giving and placing (The Latin word "dare" means "to give"). You can see the difference between "in den Raum" (into the room -> directed) and "in dem Raum" (inside the room, location). But in practice you will have to learn the case along with the verbs.