I am completely self taught in photoshop apart from a few tutorials on the internet. The best way I can suggest you start learning a program such a photoshop is open a new file and mess around with it, pick colours brush, gradients, new layers etc. Until you have a basic overview of the program. If you want to know what something does or how to do something google it and I am sure you will find a tutorial for it
Main tools for skinning though are
- Lasso and marque for simple selectiongs
- Pen tool for more complex shapes (harder to use than 2 above though)
- Magic wand for selection of sponsors logos
- Paint brush, Paint bucket for filling in sections of colour
- Gradient tool for gradients obviously.
- Eraser for rubbing out detail
- Free transform is also useful for re-sizing logos.
Also you will need to know how layers work and be able to create new layers, switch them on and off, re order, delete, duplicate.
Looks really complicated when put like this but stick with it for a few months until you are comfortable using PS then you will be able to produce some good skins.
One final tip: Be patient! With learning photoshop and the tools, it is a big program and it can do almost anything. Also be patient when creating skins, for a good looking skin, which may look simplistic, can still take a few hours minimum to create.
Hope this helps.