Every sentence has a subject and a predicate. The subject is the person, animal or thing that is doing the action. The predicate usually (but not always) begins with a verb (action word), and is the rest of the sentence. To find the subject, look for the verb and then ask who or what is doing the action.
Example: The world revolves around the sun. “Revolves” is the verb. “The world” is the subject (it does the action), and “revolves around the sun” is the predicate (it starts with the verb, “revolves”).