This Russian grammar video is a very basic introduction to nouns in the nominative case. Nouns in the nominative case are the version of a noun that you will find in a Russian dictionary. Nouns retain their nominative ending or case when they are used to answer the questions:
Кто? (who?) and Что? (what?)
A few examples with very simple sentences would be...
Это моя сестра.
Russian for Free has some grammar tables demonstrating the only time you'll change the ending in the nominative case is for plural nouns.