The answer is very simple, to write on a particular topic or subject is on the basis of your knowledge. It's the person decision to write on a particular topic whatever he knows about it.
First he himself should be very confident about the topic he is choosing like he should ask himself about the topic, if he is sure he can write on it.
Second, he should be very realistic on that, should search and keep updating on the facts he is going to write on it. If you know then you can write.
Third, before start working on it make sure that whatever you are going to write is basically the work you are sharing to audiences and by reading that they can taking interest on it..
It's easy to start the things but you should know and work on the little things before going to write.
Another thing is that you can have the opinion on i.e. the self opinion so it's easy for you to interpretate the things you can relate yourself with the topic you choose so that it's going to be easy….
Choose the topic wisely on which you can write on. Give the basic points to make it more easy….
So that's the basic thing's you should keep in mind to decide what topic or subject to write on.