Learn Russian: Это первый день.

Just like in English where the word one becomes first, Russian uses a completely different word for first (первый) compared to one (один). Words that tell you the order of things (like first, second, third) are called ordinal numbers.

Why does первый end in -ый?

Ordinal numbers in Russian act exactly like adjectives. This means they change their endings to match the gender of the noun they describe. Because день is a masculine noun, we use the regular masculine adjective ending -ый.

The word день ends in a soft sign (ь). How do I know it is masculine?

Nouns ending in a soft sign can be either masculine or feminine, so you just have to memorize their gender as you learn them. День is a very common masculine word, which is why it is paired with the masculine word первый rather than the feminine первая.

Where is the verb in this sentence?

Russian skips the verb to be in the present tense. The word Это can mean this or this is. So, the sentence literally translates as This [is] first day.