The boy laughed at beggar.

The beggar was laughed at by the boy.
Given sentence is in Past indefinite (Past simple) tense and it is in the active voice. To change it into Passive voice Object (the boy) will become subject and subject (The beggar) will be object. We also use helping verb of past simple tense was with V³ form of the main verb. Keep it in mind that the preposition at must be retained with the verb.

Rule :
Subject + (was /were) + V³ + Other Agents.