Learn Russian: Это новый свитер.

When you are introducing or pointing something out using the phrase 'This is...', you always use the fixed word это, no matter the gender or number of the noun. If you wanted to say 'this sweater' instead of 'this is a sweater', you would use the masculine matching word этот (этот свитер).

Why does the adjective новый end in -ый?

Adjectives always have to match the noun they describe. Because свитер is a masculine, singular noun in the nominative case (meaning it is the main subject), the adjective takes the standard masculine singular ending -ый.

Since this lesson is about plurals, how would we change this to say 'These are new sweaters'?

You would say Это новые свитеры. The introduction word это stays exactly the same for plurals. However, the adjective changes to its plural form новые and the noun becomes свитеры.

Where is the verb 'is' in this sentence?

In the present tense, Russian almost always drops the verb 'to be' (am, is, are). You just put the subject (это) directly next to what it equals (новый свитер).