Do you put a form of estar before a prepositional phrase in Spanish?

Sometimes, but not always (Just like english).

For example.

"Quiero estar contigo" (I want to be with you) has "estar" before the prepositional phrase ("contigo": With you).

But with

"Hay un gato debajo de la mesa" (There is a cat under the table), the prepositional phrase ("debajo de": under the) is not preceded by any form of "estar".