The most important part is to figure out what you want to accomplish by writing it. If you know for what purpose you write, you'll find a topic to talk about. Purposes brings art and creature with them. Once you have a purpose you'll be fed up with ideas and sentences will go out of your fingers immediately.
Let's think about it a little more.
If you're planning to write a scientific paper, you can look through other's articles and references were made in them. Generally, what scientific writing is, to use either experimental data, or historical perspective of science to explain something in a better way. It means for both case, you have lots of data to decide.
On the other hand, fiction literature is about life generally and it is harder to come to conclusion in selecting topic to write about. But for me, I see writing as seeing ability. What you write is based on what you see. And you can look at everything at once, but you'll always see only a few of them. You can't see Mona Lisa and the nature behind at once. You should focus on only one thing among the objects and phenomena you see. For example, if you see two people fight with each other, you'll agree with at most one of them. Or you would basically in a side which defend not to be a part of that. Even if you decide not to be in one side, this means you are in side again. You have feelings, emotions, opinions about a specific event or phenomena. And as a fiction writer the best thing you can do is to show the real version of you. As soon as you become frank, not depending on how much good you are people accept you in this way. It is important because the essential part of a writer's success is neither how popular he/she is, nor his/her works are read by thousands of people. The critical part of a writer's success is weather he/she is going to be accepted by public. If readers accept you, they will be willing to read all the works you write, even if you are bad.
I stop the philosophy here.
I leave the following questions which I believe can help you to find who you are, and therefore can help you to find best topics that can explain you a little more:
Which is more important to you: human or life?
Feelings and rationality, which one do you give more precedence?
Are moral rules for human beings or people exist to obey the rules.
How do rules have to be defined? And whom they have to defined by?
What will your writing activity change in the world?
Do you write for yourself or you want to change some other things?
Look around yourself. What don't you agree with?
What are you enemy of? What do you want to fight among people's behaviours and organization's decisions?
What do you love to read/watch?
What do you have? What don't you have?
What do you think is left in the market/sphere you'll write?
Which value can you add to humanity or the world or something else?
Which problems exist and which of them can you solve?
What is your general purpose in your life?
What problems do you have?
What is your professionality and what is your opinion about that?
What do you hide from yourself? What are the things you are afraid to tell yourself?
Does the justice exist in life?
Did you have a depression? What was the breakpoints you observed? Can you help people have the problems you had before?
Is life difficult?
Rustem Azimov
Writer of 20+ story, 150+ essays, 2 little romans in Azerbaijani.
A laboratory assistant in Control Systems Institute Azerbaijan National Academy of Sciences in the field of Intelligent Systems.