Seek out this article:
A Measure of Media Bias., By: Groseclose, Tim, Milyo, Jeffrey, Quarterly Journal of Economics, 00335533, Nov2005, Vol. 120, Issue 4
It's available through EBSCO, which you can probably link to through your local library's Web site. I think you'll be surprised just how much scholarly work has been done on this, and just how egregious the results are.
There is, in fact, media bias -- Left and right. However, the number of right-leaning news outlets is grossly outnumbered by the left-leaners. And, the lefties tend to be much more mass market (mainstream TV networks, large city newspapers) than the righties (cable news, a couple of papers in major political centers).
ABC, NBC, CBS, the NY Times, the Wall Street Journal (#1!), the Washington Post, and the LA Times are the most left-leaning news outlets in the country. They're also the most accessible and therefore the most watched and most read.
Bias goes both ways, but the fact is that an awful lot of people who go into the news business do so because they "want to make a difference." Interestingly, the "difference" these people want to make is often one with a left-leaning agenda.
Anyone who's been to college has met journalism majors who dress like hippies and spend half their lives eating tofu and attending meaningless protests. These are the reporters of tomorrow; or, in the case of people like me who went to school ten years ago, they are the reporters of today.
Bias is real and measurable. Read the above article. It's from a reputable, peer reviewed journal and contains a lot of very good information. If nothing else, you'll learn a bit about the distribution of bias in US media today.