What will your film be about? Your documentary should be worthy of your audience's time (not to mention your own). Make sure your topic isn't something mundane or universally agreed-upon. Try instead to focus on subjects that are controversial or not-well known, or try to shed new light on a person, issue, or event that the public has largely made its mind up about. In simplest terms, try to film things that are interesting and to avoid things that are boring or ordinary. This doesn't mean your documentary has to be huge or grandiose - smaller-scale, more intimate documentaries have just as much of an opportunity to resonate with an audience if the story they tell is captivating. Try out your ideas in verbal form first. Start telling your documentary idea in story form to your family and friends. Based on their reaction, you may do one of two things; scrap the idea completely or revise it and move forward. Though documentaries are educational, they still have to hold the audience's attention. Here, a good topic can do wonders. Many documentaries are about controversial social issues. Others are about past events that stir up strong emotions. Some challenge the things that society views as normal.  Some tell the story of individual people or events to make conclusions about larger trends or issues. Whether you choose one of these approaches or not, make sure you pick a subject with the potential to hold an audience's attention. For instance, it would be a bad idea to create a documentary about everyday life in a random small town unless you're really confident you can make the lives of ordinary people interesting and meaningful in some way. A better idea would be to make a documentary that casts the daily life of this small town against the story of a grisly murder that took place there, showing how the town's inhabitants were affected by the crime. Good documentaries almost always have a point - a good documentary may ask a question about the way our society operates, attempt to prove or disprove the validity...

TL;DR in Indonesian:
Pilihlah topik yang layak ditonton. Carilah topik yang menarik bagi Anda serta memberi pencerahan dan membuat penonton Anda tergerak. Ciptakan tujuan untuk film Anda. Lakukan riset mengenai topik Anda. Tulislah garis besar topik.