Learn Spanish (Latin America): Hay que buscar a la policía en la calle. - You have to look for the police on the street.

Hay que is used for impersonal obligations, meaning "one must" or "people in general have to". English often uses a general "you" to express this same idea. If you specifically meant "You (the person I am talking to) have to", you would use tienes que.

Why is there an a after buscar?

This is the "personal a". In Spanish, when the direct object of a verb is a person or a group of people, you must put an a right before it. Since you are looking for the police (people), it has to be buscar a la policía.

Does la policía mean a female police officer or the police force in general?

Here, it refers to the police force in general. As an institution, it is always a feminine noun (la policía). If you wanted to talk about a single male police officer, you would say el policía.

Why is it en la calle when we say "on the street" in English?

The Spanish preposition en can translate to "in", "on", or "at" depending on the context. Spanish speakers say en la calle (literally "in the street") where English speakers normally say "on the street".