Learn Italian: Io sono il ragazzo.

You can absolutely just say sono il ragazzo! Because the verb sono already means "I am," the pronoun io (I) is usually dropped in everyday Italian. You only include io when you want to add emphasis, like saying "I am the boy (not someone else)."

How can I tell that ragazzo is a masculine word?

In Italian, singular nouns that end in -o are usually masculine, while those ending in -a are usually feminine. Because ragazzo ends in -o, it is masculine. If you wanted to talk about a girl, you would change that ending to an -a, making it ragazza.

The lesson covers un and una, but this sentence uses il. What is the difference?

Un means "a" or "an" (an indefinite article), while il means "the" (a definite article). You would use un ragazzo to say "a boy," but this sentence uses il ragazzo because it specifically means "the boy."

Does the verb sono change if a female speaker says this sentence?