Clothes design starter tips
From Second Life Wiki
Your Second Life Avatar starts out with enough clothing and fabric choices create a number of different outfits and looks to suit your mood -- and Second Life's many clothing boutiques are always open and ready to help you expand your selection. Eventually, you might want to create your own, one-of-a-kind design. Maybe you've got an eye for fashion and want to enter the competitive world of haute couture. Second Life provides you with all of the tools you need to be your own favorite designer.
Your career in high fashion starts with the Second Life Fashion Design Guide and Templates, available under SL Knowledge Base Category:Guides
Using the Templates is step-by-step guide to using the Second Life Fashion Design Templates, a series of easy-to-use Photoshop template files corresponding to different parts of your clothing and appearance -- available for download along with the Guide, to create custom clothing textures. Designer clothing for your Second Life Avatar requires access to and a basic understanding of Photoshop, Paint Shop Pro, or a similar paint program capable of opening Photoshop .psd files.
Another resource is the Second Life Design and Texture forumsYou can ask other designers questions and look for resources.