Hi there,
This is a very broad question that covers many areas of tasks.
First thing to do is to practice with your software you will need to be able create selections,
crop and resize images, and cut and paste among other tasks.
Also familiarize yourself with working with layers as well as file formats.
But in a quick and dirty nutshell for making a pair of pants, visualize doing a RL cut and paste
job on a paper doll with a front and a back.
Often you wont be able to find exactly the right front and back for your pants.
This brings up the SL template, go to your account page on the SL website for example,
and on the lower left is the downloads link, there you can get the templates you will need.
I believe for pants you want the lower body template.
In my humble opinion actual rl materials that are scanned or shot with a digital camera are superior to most things you will find on the net.
If you can get a full shot of the front and back that works for you great. However often
you will need to do the pockets and zipper and cuffs separately.
In which case you will cut these portions out and paste them into their approriate areas.
One tip I have heard often is to make sure that your material covers the edges of the template sufficiently.
Also you will want to keep your edges uniform as well so that your seams line up well.
These comments are intended to give you an overview of what you will need to do.
I would definitely read the stickies if I were you at the top of this page.
Robin Soujourner and Chosen Few have some excellent tutorials in those that are invaluable to the SL clothes creation process for new learners.
I hope this points you in the right direction.
