You can always use the gimp, and it's free. It'll give you enough basic tools and it's very similar to
what you can do with photoshop.
Think of a concept. What I like to do when I make a new wallpaper is to think of a phrase or moment of the anime or
character. I think of what colours, what type of texture will reflect this mood.
Then I compose. Look up "aureous section" and "rule of thirds", as these will be your best friends
in wallpapering.
Keep several versions of your layered creation. Work with different fonts (and do get lots of new fonts to work with
them). Don't over-effectualize. Photoshop effects are cool and all, but using them too much looks cheap.
And have fun.
* Frisbeetarianism is the belief that when you die, your soul goes up on the roof, where nobody can retrieve it.*