Learn Croatian: Novi automobili su brzi.

They are agreeing with the noun automobili (cars). Since automobili is a masculine plural noun in the nominative case, any adjectives describing it must also take the masculine plural ending, which is -i.

In English we say 'New cars are fast', not 'fasts'. Why does brzi change its ending here?

In Croatian, adjectives must match the noun they describe in gender and number, even when they are separated by the verb 'to be' (like su). Because automobili is plural, the adjective brzi must also be plural.

How do I know automobili is a masculine plural noun?

You can tell by looking at its singular form, automobil. Because the singular form ends in a consonant, it is a masculine noun. To make most regular masculine nouns plural, you simply add -i to the end.

Could I start the sentence with su?

No, su (are) is a 'weak' word called an enclitic, which means it can never be the first word in a sentence. It naturally goes in the second position, right after the phrase Novi automobili.