Probably the most common think you ask when you first start designing for apparel is how to create halftones so that you can design with some range of gradients and shading on the ones that beg for it. Unfortunately there is no magic halftone button in Photoshop yet, but it turns out it's not that hard to do.
A heads up, this is not a very "official tutorial" just how I go about it from what I have learned. Also I use a PC, not a mac. I think you substitute Ctrl with Cmd on a mac. If you have better ways or tips n' tricks please leave a comment.