Learn Italian: Il ragazzo spagnolo è molto giovane.

In Italian, descriptive adjectives, including nationalities like spagnolo, usually go right after the noun they describe. That is why we say ragazzo spagnolo instead of spagnolo ragazzo.

Why is there an accent mark on è?

The accent mark shows that è is the verb meaning "is". If you write it without the accent (e), it means "and".

If we were talking about a Spanish girl, would giovane change to giovana?

No, it would stay exactly the same! Adjectives that end in -e in the singular, like giovane, are used for both masculine and feminine nouns. You would say la ragazza spagnola è molto giovane.

Could molto change its ending here?

Not in this sentence. When molto means "very" and is used right before an adjective like giovane, it never changes its ending. It only changes its ending when it means "many" or "a lot of" and is attached to a noun.