What is the preposition in the sentence there must be a cloud over your head?

Prepositions are Words that represent where something is in relation to something else. Think of standing on a bridge...anything describing where something is in relation to the bridge is a preposition. On, under, beside, near, etc.

In this example "over" is the preposition.

The prepositional phrase continues until you get to a noun (subject), so in the example above "over your head" is the prepositional phrase.