Sometimes you just need to be in the right mindset before you can pay attention. Before class starts, try looking over your homework, reading from your textbook, or looking at the previous day's notes. This can get your brain into "class mode", so that you can have an easier time focusing.  Preparing yourself by getting everything that you'll need and arranging your desk can also help you focus. This will cut down on distractions, like needing to borrow a pencil because yours is out of lead. By changing your environment, or the things around you, you can actually really help yourself concentrate. This doesn't just mean getting rid of distractions, although that can certainly help too. Just sitting in a different spot may help you focus by changing what you can do from where you sit. Sitting in the front, for example, can help you focus because you'll know the teacher is watching you. Sitting away from your friends can also help, because you won't be physically able to talk with them as much. 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. Taking notes can help you focus on what your teacher is saying, even if you don't think you'll need the notes to study from later. If you can use the notes then even better! As your teacher talks, make an outline with a few side notes on the really complex topics. You'll find yourself focusing better before you know it. If you don't know how to take notes, we've got you covered!...

TL;DR in Indonesian:
Persiapkan diri sebelum masuk kelas. Cari lingkungan yang baik bagi Anda. Berpartisipasi di dalam kelas. Buatlah catatan. Lakukan penelitian tambahan. Mengembangkan rutinitas.