You can write about pretty much anything you like.  Writing reviews is one of the most popular things, however you can also do fan pages, guides and how to do something, recipes, informative articles about any thing you know or interests you like hobbies, holidays, stories, top tens, top five etc.  

The only thing you need to be aware of is you could get in trouble for writing untrue or very negative things about a person or company online.  Where ever you decide to submit your articles to, need to have their terms and conditions read thoroughly by you to make sure you abide by all the guidelines and rules.  That will help protect you from getting into trouble.  

I have found some places where you can post various things from articles to opinions and obviously get paid a little.  Don’t forget to look at the writing reviews section as well.