You cannot acquire copyright protection in short phrases. While you are generally free to put whatever saying you want on a shirt (a free speech issues), you need to be careful, and as my colleague noted, that the phrase you are using is not being used as a trademark especially if it describes a similar good, like clothing in your case.
Further, some sayings may draw a false association with a person, company or brand. In such a case, they may have claims under their right of publicity or privacy and/or misrepresentation.
You can certainly seek your own trademark registration, but of course this may not be practicable if you plan on using a number of phrases because a TM identifies a good or service in commerce and is not merely a saying. I will link you to a description of the TM Due Diligence below and you may want to discuss further with a lawyer in private.
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All artwork is copyrighted at the time it is created. Registering a copyright usually only helps you to prove that you created the artwork first in case you want to go after another party for infringement Some slogan based designs can and are trademarked. If your design is more slogan/text based, you may want to look into registering a trademark instead. Keep in mind that not all t-shirt slogans can be trademarked, as trademarked are usually reserved to protect "brands" rather than protecting a "slogan". If you can prove your slogan is your brand, I think you may have a better chance at securing a trademark.
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You can't. It is likely legal. You don't need to and they aren't. Short slogans can't be protected by copyright. They have repeatedly been held not to have the requisite originality and creativity. They might be trademarked if not generic or merely descriptive or likely to be confused as to source due to use of that phrase.
You show you are an utter novice when you use "copyright" and "copyrighted" as verbs. Utter novices need to use copyright attorneys so utter novices don't get utterly screwed by what they unwittingly do.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.