Learn Italian: Il panino è buono. - The sandwich is good.

In English, we borrowed the plural Italian word panini and use it to mean just one sandwich. But in Italian, panino is singular (one sandwich) and panini is plural (two or more sandwiches). Since this sentence is about a single sandwich, it uses panino.

Why does the word è have an accent mark over it?

The accent mark is crucial because it tells two identical-looking words apart. With the accent, è is a verb meaning "is". Without the accent, e is a conjunction meaning "and".

If I were talking about a pizza instead of a sandwich, would I still use buono?

No, you would use buona. In Italian, adjectives must match the gender of the noun they describe. Il panino is a masculine noun (ending in -o), so it uses the masculine adjective buono. Because la pizza is a feminine noun (ending in -a), you would change the adjective's ending to match: La pizza è buona.