same way you do it in real life. apply for a trademark via u.s. patent office or trademark office. here is a link..

http://www.uspto.gov/tra...started/trademark-basics

Believe it or not, the vast majority of the people who might put a TM or C next to something in world, and claim to have trademarked or copyrighted it... didnt.

But if you do it, for real, thats great!

TM you dont have to register a TM(well the fact is you just cant or it would be the R symbol for registered) is not the same as ® and means your using a name but the name is not registered. Why ® is for Registered~

Far as copyright once you make/produce/record/write something you have copy right on that whether its registered or not and no need to register it but you can of course if you feel the need but your not any more protected but can make protecting it easier much like an ® that is registered~ Regardless if its registered or not you still are the copyright holder and you have a copyright by default on your works and nothing else is legally needed
so make a pretty pixel shirt or sing a little song or make a pretty poem and use that Circle C~

You can register your copyright by filing a simple form and depositing one or two samples of the work (depending on what it is) with the U.S. Copyright Office. There are different forms for different types of works -- for example, form TX is for literary works while form VA is for a visual art work. http://copyright.gov/eco/

The other thing to keep in mind if you trademark something is that the laws are completely different than they are with copyright. The major difference is that with Copyright, you can know about an infringement and choose not to prosecute the offender, typically if they're so small time that there's no way you'd even recoup the attorney costs to go after them. With Trademark you really don't have that option. If you know about it or hear about someone using your trademark, the case law says you *have* to go after the offender, or risk losing your trademark, forever. That's one of the main reasons Disney got such a reputation for going after anything and everything. People thought all of those lawsuits were copyright infringement cases, and they were that too. But the main thing Disney was trying to protect in most cases was their Trademarks. And they really didn't have a choice in whether to go after the offenders or not. cuz no way were they going to allow their numerous Trademarks to be confiscated by whomever wanted to grab them.