A complete phrase would be "to prepare X for Y." X is the thing that is being prepared. Y is the reason why it is being prepared. "To prepare the car for the trip, I am filling the tank with gas." "To prepare the vegetables for cooking, I am cutting them into small pieces. If we leave out X, it is understood that X means "myself" (or "himself," "ourselves,") Our grandchildren are visiting this after noon, we must prepare for their visit by buying groceries. That is, we must prepare [ourselves] for their visit. Dogs prepare for winter by shedding their light summer coats. That is, dogs prepare [themselves] for winter by shedding their light summer coats. We can also leave out "for Y" when it is clear without saying it. "It's time to prepare the food." The (usual) reason to prepare food is to prepare it for eating, so we don't need to say "it's time to prepare the food for eating." "I need to prepare my speech." A speech is (usually) for presentation, so we don't need to say "I need to prepare my speech for presentation."