Learn Italian: Il fiore è giallo. - The flower is yellow.

Nouns ending in -e in Italian can be either masculine or feminine, so you often have to learn the gender when you learn the word. However, in this sentence, the article il (the masculine word for "the") and the adjective giallo (which takes the masculine singular -o ending) give away that fiore is a masculine word.

How would I change this sentence to say "The flowers are yellow"?

You would say I fiori sono gialli. The article il changes to i, the noun fiore changes to fiori (because nouns ending in -e change to -i in the plural), the verb becomes sono (are), and the adjective giallo changes to gialli to match the masculine plural noun.

Why is there an accent mark on the word è?

The accent mark shows that it is the verb meaning "is" (from the verb essere). Without the accent mark, e just means "and". The accent is crucial in writing because it completely changes the role and meaning of the word in the sentence.