When I find myself wanting to write but don't know what subject or topic to write about, I look at my surrounding. I look at what others are doing or even try to listen to some conversations which I found having sense, and try to start writing about those things. Sometimes, I look at the newspaper and will try to find a word or group of words that I can start a topic with. Example, my article about breast cancer (which you can find here: https://hubpages.com/health/Breast-Cancer-What-to-... ). I was reading that time a newspaper and in the obituary, I saw someone who passed away because of it. I began researching about it and I wrote down all what I have learned and even my own knowledge about it.
Hope it helps.