hi & welcome,
desiging clothes is done mainly with your choice of paint programmes, using a template for desired part of clothing,head,upper body, lowerbody,skirt,, etc,
Gimp being a popular and free open source paint programme,
Here is a list of some useful links with some great tutorials on them as well click me for stuff
their are some helpful tutorials given in second life as well, search inworld for Builders Brewery
good luck & happy travels