Please read the tutorial:
http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/UV_Map_Basics.
Short version: finish editing the untextured model in Blender, then use a
3D View window to mark seam edges (with the edges selected,
Ctrl-E (
Edge Specials) ->
Mark Seam) and make an UV map (
U (
UV Calculation) ->
Unwrap or one of the other variants); use a
UV/Image Editor window to see and edit the mapping. Repeat until you get the seams and the unwrapping right.
At this point you have the "skeleton" of your texture mapping; save a draft texture (
UVs ->
Plugins ->
Save UV Face Layout...) and choose a size and a filename.
With this reference you can use Blender's builtin image editor (
Image ->
Texture Painting) or not (
Image ->
Open... and
Image ->
Save) to edit your texture.
As soon as you have a meaningful texture, you can use it to render your model: enable
TexFace in the relevant material.