The verb 在 (zài) expresses existence in a place, similar to the English "to be located in".
The verb 在 is used to express existence in a place. English does not have a verb that covers this, and instead uses "to be" with a preposition. In Chinese, 在 covers both of these.
subject + 在 + place
Remember that you don’t need another verb in this construction. It can be tempting to try use 是, as English uses "to be", but this is not correct. 在 is the only verb needed.
我 在 上海。