Okay I found a nickname for a group of people that is becoming very popular. There is a lot of online stores selling this name or clothing as a logo, some of them even have it as part of their name for their sites online. But no one has put a trademark on it. What happens to them if I trademark the logo and name and start selling the clothing on my own.
Does it matter if they were using the name before me? Does it matter if we use different letter fonts or colors?

I really don't think you can use it since it is known as theirs by most people. I don't know if any of these lawsuits would answer your particular question.

Polar Beverages - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Polar_Beverages#Conflict_with_Coca-Cola)

Victoria's Secret Sues Macy's; Naomi Campbell Flies Again -- The Cut: New York Magazine's Fashion Blog (http://nymag.com/daily/fashion/2008/04/victorias_secret_sues_macys_na.html)

Judge rules in 10-year-old Victoria's Secret case - U.S. business- msnbc.com (http://www.msnbc.msn.com/id/24760241/)