Registering a Trade Mark requires that you use that mark "in commerce" first. When ever you apply for a trade mark, the US Patent Office requires that you show the mark used in commerce. In your case you would need to show that mark on a t-shirt or clothing hang tag or something. Trademarks are usually reserved for company logos, slogans, or product logos and that type of thing.
Your individual t-shirt designs should be copyrighted. I think you can copyright your designs relatively inexpensively, but I can't remember how much it costs.
I have heard of a way of copyrighting your designs that is practically free. I don't know if this works but....I was once told that you could put all your designs in an envelope and mail them to yourself. But don't open the envelope....just keep it someplace safe. The idea is that if you ever had to prove that the designs were yours and when you created them, you could use that unopened letter. Basically, you have a postmarked letter (with the date on the postmark) that shows when you created the designs because the letter is sealed and date stamped from the federal government (the US Postal Service). Again, I don't know if that would work well.....but I have heard that it helps if you have to protect your designs.
If I were you, I would start by creating some of your t-shirts and a real simple catalog to help you promote your designs. Keep the catalog simple and low cost....but definitely include pictures of the t-shirts and not just drawings......pictures always go a little farther to selling your ideas (especially if you don't have any samples with you). Good-luck, and remember to have fun with it!
Do I need a Sample T-shirt? 
