Participate in class. Participating in class can also help you focus. Participation keeps your mind engaged and focused on the lesson because you can't just drift off or think about something else. Anything you can do to participate, from asking questions to getting in to group projects or discussions, can really help you.
Ask questions. A good way to participate in class is to ask questions. When you have a question about something you don't understand or the teacher said something and you want to know more about it, raise your hand and ask. Even just focusing enough to listen for things you might want to ask questions about can help you pay way more attention.