Well, before you even start slapping logos down, what's your vision? Are you seeing yourself as a casual line, trendy, or more business oriented? As soon as you have a targeted area, then you'll be able to have a foundation from which to explore.
How do you plan to produce these shirts? Silk screening? Press ons? Yourself, or through a third party?
As far as checking patents, this site may help you:
http://usgovinfo.about.com/gi/dynami...t.gov/records/