You'll need a photo editing application in order to edit the JPEG texture files. Photoshop is the preferred choice, but GIMP (
http://www.gimp.org/) is free and almost on the same level as Photoshop.
As for the actual editing of textures, once you've learned how to use your program of choice, you've pretty much learned how to edit textures. There are many great tutorial sites for image editing and texture creation on the net, your best chance is seeking those out.
I'm sure people will be happy to give you tips and tricks once you get some WIPs going, but the general learning, you'll learn better if you teach yourself and experiment.