The nominative is used for the subject of a sentence whereas the accusative is used after certain verbs, prepositions and some other words. In English it is usually the direct object.

Er malt ein Bild.

Sie schreibt einen Brief.

The subject of a sentence should be in the nominative case.

For example

I think....ich denke

he wants.. er will

Ich and er are in the nominative case.

Other then the nominative you also have the genitive (which is mainly used to show ownership, so I would ignore it for the moment), the dative case and the accusative case.

Different verbs will either take the accusative or the dative (or both) and different prepositions will to. You generally need to learn these. Although the accusative is more common.

Examples

Er hat den Ball (den is nominative)

Ich will eine Tasse (die is accusative, but you can't tell as the accusative and the nominative are the same for feminine objects)

As a general rule of thumb I would use accusative for the second noun, until you start learning the dative in class.

I like this website for learning German: https://yourdailygerman.com/german-cases-nominative-genitive/ Hope it helps.