Review your notes from the previous class beforehand.
Preview the material for today's lecture. If your teacher follows the textbook, take a look at vocab words and the headings in that chapter. If your teacher posts notes or powerpoints online ahead of time, read through those and print them off.
Shower and put yourself together - get dressed, brush your hair, wash your face and whatever else you like so you don't show up to class looking like you just rolled out of bed.
Eat breakfast or lunch beforehand. You won't be able to concentrate in class if you're just thinking about what you're going to eat after class, and nobody else will be able to concentrate in class if you're eating during lecture (especially if it's something good).
Arrive 5-10 minutes early to class.
Have your supplies up and on the table/desk. Get out your pens/pencils, notebook, folder, highlighters, laptop - whatever you use in that class!
Eliminate distractions. Put away your cell phone, close out of your facebook/twitter/whatever tab.
Be ready to learn. When you're in the lecture haul, it's time to focus. At the very least, respect that your teacher is in front of you, sharing their knowledge with you and your classmates. The best way to respect that then to pay attention - look at your teacher, take notes, keep your phone in your bag, and ask questions if you have any!
I hope this post helps you out and has better explained what it means to "be prepared for class." If you have anything else you would add to this list of what it means to be prepared, let me know in the comments!