If it starts to luff, you have two choices: tighten the sail sheet until it stops luffing, or steer away from the wind (bear off). When the sail luffs, it means that you are heading too much into the wind for your current sail setting. If you bear off slightly, (away from the wind) your sails will stop luffing. If you see it change so that the wind is coming from a direction that is more behind you, you will be wasting energy. Let out the sail until it is perpendicular to the wind. You will be constantly watching the sails, the telltales, and trimming sails because the wind won't blow from a constant direction for long.  When the wind is at your back and side (aft quarter), it's called a broad reach. This is the most efficient point of sail as both sails are full of wind and pushing the boat at full force. When the wind is at your back, you are running with the wind. This is not as efficient as reaching, because air moving over the sail generates lift and more force than just the wind pushing the boat. When running with the wind, you can pull the jib over to the other side of the boat where it will fill. This is called wing-on-wing, and you have to maintain a steady hand on the tiller to keep this sail configuration. Some boats have a "whisker pole" which attaches to the front of the mast and the clew of the jib which makes the jib much easier to control and keep full of wind. Be sure to be vigilant of obstacles and other vessels, as having both sails in front of you blocks a significant portion of your view.  Be careful—when the boat is running, the sails will be way off to the side, and because the wind is basically behind you the boom can change sides suddenly (jibe or gybe), coming across the cockpit with quite a bit of force. If you have a wind direction indicator at the top of your mast, do not sail downwind (run) so that the wind indicator points toward the mainsail. If it does, you are sailing with the boom on the windward side (sailing by the lee) and are at high risk of an accidental jibe. When this...

TL;DR in Indonesian:
Perhatikan sisi layar bagian depan pada layar utama atau mainsail dan jib. Perhatikan indikator angin Anda (telltales). Close reach. Close haul. Berlayarlah sesuai arah angin untuk mencapai tempat tujuan. Bersikaplah tenang ketika belajar. Berlayarlah dengan aman.