If you are writer or hobbyist it's easy to write a hub. For everyone who starting out with hub I'd recommend first to navigate site, and read FAQ, forums and other hubs. There is plenty of questions and ideas what you can write about. Of course if you do not know anything about certain subjects hub will be not what readers expected. Visitors comes to hubs searching for all kinds of tips.
If you have knowledge of particular subjects and you want to share with others this is where you start. Taking photos, and post with your hub will help visitors as well to have visual feelings about your subject.
Let me ask you what do you know the best?