Most people in Second Life find themselves purchasing from shirts already available within Second Life. In order to create shirts of your own, a user must create the proper image and upload it to Second Life.

There are templates and information for this available at the Second Life website.

To aide in the process of creating your own shirt, we created 2nd Shirts. With 2nd Shirts, you can upload an image on the site, select a few simple options, and 2nd Shirts will quickly generate a new image that will be ready to upload to Second Life and be worn by your avatar.

Getting your new image into Second Life is as easy as reading about to get your clothing inworld.

There's plenty of additional help at the Second Life Knowledge Base, including Content Creation.