When looking for a topic there are a few things you'll want to consider. One of them is making certain it is a topic with which you won't get sick. You'll be spending a lot of time reading/researching your topic so if it's not something you're passionate about, then you'll probably get board/angry with it.
When trying to decide on your topic consider these questions: What would be my purpose? Why am I writing on that subject to begin with? What is it I want to know about this topic?
The other trick is using a topic that is neither too narrow, nor to general.
For example, the topic of Media Violence is simply far too large to research without refining my search for information. There are too many aspects of media and of violence to consider. I have to narrow it down. If I wanted to focus on Media Violence and teenagers, that would give me a focus in which to direct my research. Even this topic may be too general. So to narrow it down even further, I might change it to: Media Violence and it's effect on teenagers. To narrow that down even further, I might limit my results to those articles/studies/essays that have only to do with video games, films, and television. Or I could just choose one of those three as the focus of my research.
If that is the topic I'd like to research*, I now have to come up with a researchable question, or assertion, to answer in my paper, like: Can Media Violence affect teens?, or Media Violence causes teens to act aggressively.
My null hypothesis would be that Media Violence does not have any affect on teenagers.
When conducting your search you should only be citing sources that you have deemed credible, have been peer-reviewed (deemed credible by other experts in the field), and that are recent. I'll only allow cited material that has been published in the last five years.
*A note on the Research Paper...
In school you're taught that a research paper is looking up information on a topic then presenting that information in a paper. Not true! You are not conducting research if all you are doing is reciting what others have discovered. This type of writing is called a Literature Review. You are simply providing a summary of information that has already been presented on your subject. A Research Paper is one that is written that summarizes the results of research, or an experiment.