Learn Portuguese (Portugal): Usa sempre o cinto de segurança no carro.

Usa is the informal tu command (imperative) for the verb usar. To form the affirmative tu command, you take the present tense tu form (usas) and drop the final s. If you wanted to be formal (você), you would say use.

The English translation says 'your seat belt', but the Portuguese just says o cinto. Why isn't there a word for 'your'?

In Portuguese, when it is obvious who the owner is, you usually just use the definite article (o or a) instead of a possessive word like teu. This is especially common for body parts, clothing, and personal items like a seat belt.

Does the verb usar mean 'to use' or 'to wear'?

It means both! In Portuguese, you use the same verb, usar, for using an object and for wearing clothes, accessories, or a seat belt. There is another verb, vestir, but that is strictly for putting on garments.

Why does sempre come after the verb? Could I say Sempre usa?