Here is a comprehensive list of tools for developers.
http://www.orbitersim.com/forum/Default.aspx?g=posts&t=4531
There you may find GIMP, which is very similar to Photoshop.
A texture is basically art. So before attempting to master a software, you should care about perfecting your understanding of arts.
The idea of how to make a tutorial for newbies on how to make textures was something that troubled me for months. But finally I reached the conclussion that a texture is basically 2D art, just like when you make a drawing on paper.
If you are completely strange to 3D modelling and texturing, I recommend to start trying to practice constructing paper models at home.
Here some paper models so you can practice.
http://www.mechwarrior3.org/wforum/viewtopic.php?t=2777&sid=3fad844dece5fe65838eadf8354a4ed9
And here is the software to view those paper models (not sure if you can print them, I do not remember)
http://www.tamasoft.co.jp/pepakura-en/download/viewer.html
When you make your own originally created paper models on paper, you get the idea of coloring so you can understand what texturing is. In the end, texturing is like making a digital 2D skin for a digital paper model.