1. Photoshop (or GIMP or Paint Shop Pro). Most tutorials are designed around Photoshop so if you have that, start there. A friend of mine designs
SL clothes in GIMP and it's free. I've never used it so have no idea how it works.
2. Some starter templates. See
http://robinwood.com for a good t-shirt template and lots of interesting stuff about
SL clothing design.
3. The freebie
SL Clothing Previewer.
http://www.fileden.com/files/2006/10...9/slcp_win.zip
This will save you lots of $L in uploads since you can tell how stuff will look and make sure seams match.
4. Either Chip Midnight's or Robin Wood's avatar UV maps. You need those to locate seams and cross clothing boundaries.
Good luck.