If you are completely new to 3d design, then Google SketchUp (http://sketchup.google.com/) or Blender (http://www.blender.org) could be easy free ways of determining if this is something you want to pursue.
If this is just for pictorial reference and not to be used for production of piece on CAD machinery, then may I suggest you consider getting a copy of "Poser" and then Google up techniques on making clothing for Poser models... Not a cheap route but you will get the whole model fitted on a human form that is great for easy pictorial posing and reference...
Short of that the free software already talked about is a great way to at least get your feet wet...
Now if you plan to have it produced on CAD equipment you will probably want to look into Autocad or Solidworks as most shops will be using those two software packages, and will allow you to better interact with the shop when you are sharing similar software packages and established formats...