The nominative case is used to describe the subject of a sentence. The subject is the main actor in the sentence, the person who carries out an action:
The nominative case is also used to describe predicate nouns (i.e., nouns that are on the other side of verbs such as sein, bleiben, heißen, werden, scheinen from the main subject):
In the first example, die Prinzessin is the main subject and die schönste Frau is the predicate noun. In the second example, Schneewittchen is the main subject and Rechtsanwältin is the predicate noun.
The biggest trick to being able to use the nominative case (and all four cases, really) is to know the gender of nouns. If you don't know whether Zwerg is masculine, feminine, neuter or plural, the cases can't really fall into place.
Definite and indefinite articles in the nominative case
Articles in the nominative case (there is no equivalent of the indefinite article in the plural, since, just like in English, you can't say "a dwarves"):
Following are the personal pronouns, which can replace nouns (and only exist in pronoun form for the referents: I and you), in the nominative case: