In order to create virtual fashion you need to be able to use some kind of graphical software. Important is that the software you choose supports layers and will be able to ready the Adobe psd files. If you can afford it, you could use Photoshop but make sure you don’t use a version older than Photoshop7 (that is the last one before the Creative Suits series: Photoshop CS1, CS2 etc). Photoshop is quiet expensive, but there is a possibility of renting the program and pay per month.
Other popular programs used to create textures and clothes are Photoshop Elements, Adobe Illustrator, programs from Corel and Procreate Graphics such as CorelDraw, Paint Shop Pro, Photo-Paint and Painter. Important when you use are going to choose a program is the capability of opening psd files, as most templates offered are saved in this format and the possibility of using .abr files so you can use the wide resource of brushes available.
A very good alternative to Photoshop is the free GIMP. In this program you will find most of the features that are available in Photoshop and yet the software is completely free to use.