German is notoriously difficult for cases and determining which case to use in which contexts. Cases are similar to conjugations of verbs in that depending on what the noun is, you have to change the article and potentially the noun itself. Let's take a look at the Accusative, Dative, and Genitive cases today.
Accusative: The accusative is essentially the direct object of the sentence. For example, if I wish to say that I eat the apple, instead of der Apfel I would have to say den Apfel. Following are the rules for accusative:
der -> den
das -> das
die -> die
For example: Ich esse den Apfel. Ich esse das Brot. Ich esse die Erdbeere.
In situations where you are saying you want to go somewhere or place something somewhere, make sure to use accusative as well. There are verbs that you must memorize that only use accusative, and following some conjunctions one must use accusative sometimes and dative others.
Ich gehe in den Einkaufszentrum. I go to the shopping mall.
Dative: The dative is the indirect object. Who are you giving the direct object to? For example:
Ich gebe den Apfel zu der Frau.
The rules are a bit different:
der -> dem
das -> dem
die -> der
If something is already in position or if you are at a location, use dative.
Ich bin in dem (im) Einkaufszentrum. I am in the shopping mall.
Genitive: Genitive is the easiest to understand. It is essentially conveying possession; something belongs to this noun. If I want to say the flag of the country:
die Flagge des Landes.
der -> des
das -> des
die -> der
In addition, if you use des, you will most likely have to add an -s to the end of the noun, as you see in des Landes.
Hope this was helpful!