If I were to design a shirt or any type of clothing with a graphic design. And I don't want anyone to steal my design and claim it as their. What should I do to prevent it?
In the United States, you can file a copyright with the Library of Congress (LOC) prior to production:
This does not prevent anyone copying the idea, but it does protect how you've implemented that idea (the artwork). It may still be lifted, but if your copyright is on file first at the Library of Congress, you have great footing for any infringement lawsuit you may wish to file.
Note that filing a copyright with the LOC is not a requirement. Artwork is copyrighted to the artist(s) the moment it is created. However, by filing with the LOC it's on "official record" as to when and who created the work, making court cases more favorable to the registered owner(s). In fact, in some instances, if your work in on file first with the LOC, you may not even need a civil court for infringement resolution.
Of course, other regions will have different standards and procedures. There are some countries which are notorious for never worrying about copyright infringement concerning the work of others.
All that being posted.. if you don't want something taken, don't put it on the internet. That's really the only way to prevent it unless you have massive funds to file many lawsuits or someone who can constantly send out cease and desist letters.
Disclaimer: This question is verging on a legal question, and since I am not a lawyer, this answer doesn't constitute legal advice. If you want legal advice hire a lawyer.
You can't really do anything to prevent someone from stealing something you have designed, especially if it's something you are publishing publicly or something you are selling to the general public. If a design can be seen then it can be stolen.
You could always lock your designs away never to see the light of day, but then nobody would ever see them.
If we could stop theft, then the police, lawyers and judges would be out of a job.
However, the law does offer some protection - it's called copyright, and there's nothing you need to do to get it. Copyright is conferred automatically to the author of an original work when it is created. Please note: that laws differ by country obviously. In some countries you can register your work for extra protection, such as the USA, but that isn't required here in the UK for example.
This is interesting question. Obviously you want to sell something and thats why you want to hear something else than "do not show your designs, then they are in safe"
Your main options are:
People do not steal your designs if you sell designs which aren't yours
People do not steal your designs if the designs are considered to have low quality
People do not steal your designs if they have a reasonable reason to be afraid of bad consequences
People do not steal your designs if they cannot use them
Option 1 is legal solution without buying any licenses if you use public domain or free to use stuff. But the idea to use that specific image, text or form can still be copied if it can be seen, so option 1 can be considered only as playing with words (=formally right, but not very useful answer)
Option 2 is in use automatically if your work is not interesting. I bet you do not want to use option 2.
Option 3 is much theoretical, because you probably have no way to affect the wellfare of people in other countries. In western developed countries you can file copyright infringement lawsuits, but that needs time and money.It needs also a proof that you made that design first.
In other countries you can belong to a clan who has its own specific means to distribute justice at least locally, but in developed western countries that's illegal.
Proofing that the design was made at first by you: You can send your designs to yourself in registered post or use available local registration services. At least one of the other answers suggest something like cryptography based binding your design, a time stamp and your claim "I made this first". Unfortunately I do not know how much proof power those modern into internet registration methods have in a court of law when some other than the prosecuting attorney tries to use them. I'm afraid it's nothing until the law know them.
Option 4 can mean something special technology which isn't still easily reproduceable. Think for example some hi-tec printing method which makes the T-shirt image actually a computer which must have some special software. I believe it will come, but when it happens, also the copying methods can be others than today
Option 4 can also mean that people who want carry your designs are some diciplined special group which buys only from some specified source. If that source buys only from you, nobody else has any use for your designs. Religions and extreme political orientations can offer that possiblity.