Mark seams on you model in blender by using ctrl e- mark seams, split your screen and open a UV editing window. Once you have marked your seams press U- Unwrap in edit mode and you will get a visual representation of your unwrapped mesh. Then in the UV editing window go to scripts and export UV. Then export your image as a targa file. You can open this in PS or GIMP. Then you can either paint or overlay images and then simply open your finished texture over your unwrapped UV mesh.
That probably sounds insanely complicated, look for the wikibooks tutorials and find something on UV unwrapping that should help you out.
However I will say that there are many different types of textures, some you may encouter are:
Colour map- A map that simply maps colour onto your mesh.
Bump Map- Uses black and white to give the illusion of bumps, while keeping the mesh the same. Used for things like pores on skin and bricks.
Normal Map- Similar to bump map, but more useful for complicated bumps, Like veins.
Opacity/Alpha Map- Uses black and white to determine which part of a texture should be transparent and which should be opaque. Often used in low res trees.
Hope this helps
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