How to make Textures for a model

I want to create textures for my model like "chair1_D.tga" "chair1_N.tga" chair1_S.tga. Please check these textures in the following link :

https://www.assetstore.unity3d.com/en/#!/content/845

I have also created a chair model, now i want to create textures like as above (Diffuse, Norm, Specular). But i don't know how can i make them, please anyone guide me. Thanks in advance.....

 

There's a large number of image editing software available out there. Photoshop is popular, but if you're on a budget GIMP is free and does most of the stuff Photoshop does. You can paint your textures by hand or bash it together with photographs (http://www.cgtextures.com/ is a great resource for royalty free textures). You could also use a program like ZBrush or Blender to sculpt a texture or a highpoly model, from which you can bake a normal map and ambient occlusion out of for texturing.

Try looking up a tutorial for a game character/object from start to finish. It should have a bit on texturing and baking textures from a high poly model.