How to change from passive voice to active voice? the tidal effects of the ocean are created by the moons orbit around the earth

You just take the verb that is in the passive form and turn it active. Also, the construction by + noun phrase becomes the subject of the active sentence, and the patient in the beginning becomes the object of the sentence. So, here is how it should be done:

The tidal effects of the ocean are created by the moon's orbit around the earth. (passive)

The moon's orbit around the earth creates the tidal effects of the ocean. (active)