We can give you help in only a general way for this question because we do not know what kind of paper you are expected to write or what your own thoughts and/or research are on this issue. I am assuming that by "thesis," you mean you need help writing a thesis statement, but let's discuss both.
A thesis is the main idea that you want to get across in your writing. If you are expected to write a paper based on your own thoughts and feelings about violence in the media, then it is your opinion on this that is your thesis. Do you think it creates more violent behavior in people? Do you think it has no effect at all? Your opinion is your thesis.
If you are expected to write a research paper, then your main idea is going to be based on the information and/or opinions you find through your research. Is there information that shows a link between violence in the media and violence real life? If so, that would be your thesis. If the evidence shows no link, you might have a different thesis.
Now, having a thesis is a good start, but a thesis statement is much more. A thesis statement states your main idea and lets the reader know how you are going to support that idea. This provides the reader with a kind of road map to your writing.
Let me give you an example. Let's suppose that I am writing a research paper and my thesis is that there is no connection between violence in the media and violence behavior. I have done my research to support that idea. Here is a thesis statement I could write:
There is no evidence of a causal relationship between violence in the media and violent behavior because, in spite of there being an upward trend in media violence, statistics on violent crime show a downward trend.
This sentence states my main idea, my position on this issue, and it tells the reader I'm going to be supporting that main idea with statistics on media violence and with statistics on violent crime.
Once you have decided what your main idea is and how you will support that idea, you can write a thesis statement with no problem.