Just the way you were taught in school: intro, evidence, and conclusion. Make sure you intro definitely answers the question and try to have at LEAST two pieces of evidence. If you can't think of another spend time elaborating the two pieces you do have.
Look at some of the current topics going on in the world of science/medicine/healthcare so you have an idea of what could be in the PCAT, too! It could be anything about effects of stress, vaccines, politics regarding the sciences, etc. So keep yourself updated.
Also, follow what piggy88 said. The essay isn't that difficult. Make sure your grammar, spelling, structure and all of that is as perfect as possible too.